fix(transforms/kompress-remote): keep compress fail-open on malformed 200 (#2320)

## Description

`RemoteKompressCompressor` (the opt-in `HEADROOM_KOMPRESS_ENDPOINT`
remote compression client) documents a fail-open contract in its own
docstring:

> Fails OPEN: any network/HTTP error returns the content verbatim so a
flaky endpoint degrades compression rather than breaking the proxy.

But only the network call and the `compressed` field check actually run
inside the fail-open guard. The metadata coercions run **after** the
`except`, outside it:

```python
try:
    resp = self._client.post(...)
    resp.raise_for_status()
    data = resp.json()
    compressed = data["compressed"]
    if not isinstance(compressed, str):
        raise TypeError("...")
except Exception as e:  # fail OPEN
    logger.warning("Remote Kompress failed (%s); passing through", e)
    return self._passthrough(content, n_words)

result = KompressResult(
    compressed=compressed,
    original=content,
    original_tokens=int(data.get("original_tokens", n_words)),
    compressed_tokens=int(data.get("compressed_tokens", len(compressed.split()))),
    compression_ratio=float(data.get("compression_ratio", 1.0)),   # <-- outside the guard
    model_used=str(data.get("model_used", self.config.model_id)),
)
```

So a hosted `/compress` endpoint that returns a 200 with a valid
`compressed` string but a malformed metadata field escapes the guard and
raises out of `compress`, breaking the proxy request instead of passing
through. The most realistic trigger is an explicit JSON `null`:
`data.get("compression_ratio", 1.0)` returns `None` for a **present**
key (the default only applies to a missing key), and `float(None)`
raises `TypeError`. A non-numeric string like `"original_tokens":
"lots"` raises `ValueError` the same way. Since the whole point of the
flag is to support arbitrary self-hosted endpoints, a slightly-off but
well-meaning endpoint (sending `null` for a field it could not compute)
takes down the request path this class exists to protect.

## Fix

Move the response parsing (the `KompressResult` construction with its
`int`/`float`/`str` coercions) inside the fail-open `try`, so any
malformed field degrades to verbatim passthrough like every other
bad-response case:

```python
try:
    ...
    compressed = data["compressed"]
    if not isinstance(compressed, str):
        raise TypeError("...")
    result = KompressResult(
        compressed=compressed,
        original=content,
        original_tokens=int(data.get("original_tokens", n_words)),
        compressed_tokens=int(data.get("compressed_tokens", len(compressed.split()))),
        compression_ratio=float(data.get("compression_ratio", 1.0)),
        model_used=str(data.get("model_used", self.config.model_id)),
    )
except Exception as e:  # fail OPEN
    logger.warning("Remote Kompress failed (%s); passing through", e)
    return self._passthrough(content, n_words)
```

No behavior change on a well-formed response; only the malformed-200
path changes (raise to passthrough).

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- `headroom/transforms/kompress_remote.py`: move the `KompressResult`
construction and its field coercions inside the fail-open `try`.
- `tests/test_transforms/test_kompress_remote.py`: add
`test_remote_kompress_null_numeric_field_fails_open` (explicit JSON
`null`) and `test_remote_kompress_non_numeric_field_fails_open`
(non-numeric string), both asserting verbatim passthrough.
- `CHANGELOG.md`: Bug Fixes entry.

## Testing

- [ ] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
$ uvx ruff@0.15.17 check headroom/transforms/kompress_remote.py tests/test_transforms/test_kompress_remote.py
All checks passed!
$ uvx ruff@0.15.17 format --check headroom/transforms/kompress_remote.py tests/test_transforms/test_kompress_remote.py
2 files already formatted
$ uvx mypy@1.20.2 --ignore-missing-imports headroom/transforms/kompress_remote.py
Success: no issues found in 1 source file
```

## Real Behavior Proof

- Environment: Windows 11, Python 3.12, `uvx ruff@0.15.17` / `uvx
mypy@1.20.2`. A full `pytest` OOMs this box (ML-stack import), so I
reproduced the control flow with a dependency-free script and left the
full pytest to CI.
- Exact command / steps: modeled the OLD (coercions outside the `try`)
and NEW (inside the `try`) parsing against a 200 body `{"compressed":
"short result", "compression_ratio": null}` and against a well-formed
body.
- Observed result: OLD raised `TypeError` on the null field (proxy
request breaks); NEW returned passthrough; a well-formed body still
compressed under NEW. The added tests assert both malformed cases
(`null` and non-numeric string) return the original content with
`compression_ratio == 1.0`.
- Not tested: a live remote Kompress endpoint; the added tests drive
`RemoteKompressCompressor` through an `httpx.MockTransport`, matching
the existing test harness in this file.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Additional Notes

The "unit tests pass locally" box is unchecked because a local pytest
run imports the ML stack and OOMs this box; the added tests reuse the
existing `httpx.MockTransport` harness in `test_kompress_remote.py` and
run under the normal CI pytest job, and the behavior is corroborated by
the standalone proof above.

---------

Co-authored-by: JerrettDavis <mxjerrett@gmail.com>

headroom/transforms/kompress_remote.py

@@ -102,19 +102,23 @@ class RemoteKompressCompressor:
             compressed = data["compressed"]
             if not isinstance(compressed, str):
                 raise TypeError("remote Kompress response field 'compressed' must be a string")
+            # Coerce the numeric/string metadata fields inside the fail-open guard.
+            # A 200 response with a malformed field (e.g. a non-numeric string, or
+            # an explicit JSON null: data.get returns None for a present key, and
+            # float(None)/int(None) raise) would otherwise escape uncaught and break
+            # the proxy request, defeating the fail-open contract this class promises.
+            result = KompressResult(
+                compressed=compressed,
+                original=content,
+                original_tokens=int(data.get("original_tokens", n_words)),
+                compressed_tokens=int(data.get("compressed_tokens", len(compressed.split()))),
+                compression_ratio=float(data.get("compression_ratio", 1.0)),
+                model_used=str(data.get("model_used", self.config.model_id)),
+            )
         except Exception as e:  # fail OPEN — never break the proxy on a bad endpoint
             logger.warning("Remote Kompress failed (%s); passing through", e)
             return self._passthrough(content, n_words)
 
-        result = KompressResult(
-            compressed=compressed,
-            original=content,
-            original_tokens=int(data.get("original_tokens", n_words)),
-            compressed_tokens=int(data.get("compressed_tokens", len(compressed.split()))),
-            compression_ratio=float(data.get("compression_ratio", 1.0)),
-            model_used=str(data.get("model_used", self.config.model_id)),
-        )
-
         # CCR stays PROXY-LOCAL: endpoint is stateless (enable_ccr=False), so we
         # store the mapping + append the retrieval marker here — same policy and
         # marker format as KompressCompressor.compress.

tests/test_transforms/test_kompress_remote.py

@@ -96,6 +96,49 @@ def test_remote_kompress_malformed_success_fails_open() -> None:
     assert result.compression_ratio == 1.0
 
 
+def test_remote_kompress_null_numeric_field_fails_open() -> None:
+    # A 200 response with a valid 'compressed' but a malformed numeric field
+    # (here an explicit JSON null) must still fail open, not raise. data.get
+    # returns None for a present key, so float(None) would blow up if the
+    # coercions were outside the fail-open guard.
+    content = _long_text()
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            json={"compressed": "short result", "compression_ratio": None},
+        )
+
+    compressor = _compressor(httpx.MockTransport(handler))
+    try:
+        result = compressor.compress(content)
+    finally:
+        compressor.close()
+
+    assert result.compressed == content
+    assert result.compression_ratio == 1.0
+
+
+def test_remote_kompress_non_numeric_field_fails_open() -> None:
+    # A non-numeric string in a numeric field is also a malformed response.
+    content = _long_text()
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            json={"compressed": "short result", "original_tokens": "lots"},
+        )
+
+    compressor = _compressor(httpx.MockTransport(handler))
+    try:
+        result = compressor.compress(content)
+    finally:
+        compressor.close()
+
+    assert result.compressed == content
+    assert result.compression_ratio == 1.0
+
+
 def test_content_router_selects_remote_kompress_from_env(monkeypatch) -> None:
     monkeypatch.setenv("HEADROOM_KOMPRESS_ENDPOINT", "https://kompress.example")
     monkeypatch.setenv("HEADROOM_KOMPRESS_ENDPOINT_TOKEN", "secret")