fix(transforms): normalize diff compressor context (#1801)

## Description

Unified diff content could skip compression when the router reached the
DIFF strategy with no question context. `DiffCompressor.compress()`
defaulted omitted context to an empty string, but explicit `None` still
crossed into the Rust boundary and raised before any compression result
could be produced. The router also had a DEBUG-only crash path because
it measured `len(context)` before DIFF dispatch. This normalizes `None`
at the router entry and at the DIFF wrapper boundary so direct and
routed diff compression both send a string context to Rust. Closes
#1798.

## 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

- Normalize `None` context to `""` before router debug logging and
compression dispatch.
- Normalize `None` context to `""` again before calling the Rust diff
compressor.
- Add regressions for explicit `None`, omitted context, non-empty
context preservation, and DEBUG-enabled router DIFF dispatch.
- Keep DIFF fallback behavior unchanged so patch-shaped content is not
routed through a lossy fallback.

## Testing

- [x] Unit tests pass (`uv run pytest
tests/test_transforms/test_diff_compressor.py
tests/test_transforms/test_content_router.py -q`)
- [x] Linting passes (`uv run ruff check
headroom/transforms/diff_compressor.py
headroom/transforms/content_router.py
tests/test_transforms/test_diff_compressor.py
tests/test_transforms/test_content_router.py`)
- [ ] Type checking passes (`uv run mypy headroom`)
- [x] New tests added for new functionality when applicable
- [x] Manual testing performed

### Test Output

```text
uv run pytest tests/test_transforms/test_diff_compressor.py tests/test_transforms/test_content_router.py -q
86 passed in 3.09s

uv run pytest tests/test_transforms/test_content_router.py -q
55 passed in 2.84s

uv run ruff check headroom/transforms/diff_compressor.py headroom/transforms/content_router.py tests/test_transforms/test_diff_compressor.py tests/test_transforms/test_content_router.py
All checks passed!
```

## Real Behavior Proof

- Environment: Windows, Python through the project `uv` environment.
- Exact command / steps: run the new DIFF context regressions against
base and head.
- Observed result: base fails explicit `None` at the fake Rust boundary
with `AssertionError: Rust diff compressor received None context`; head
passes explicit `None`, omitted context, non-empty context, and
DEBUG-enabled router dispatch.
- Not tested: native Rust internals beyond the Python wrapper boundary.

## 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
- [x] 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
- [x] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Additional Notes

No changelog entry is needed for this narrow wrapper and router bug fix.
Type checking was not part of the focused local validation for this
Python-only change.

headroom/transforms/content_router.py

@@ -1449,6 +1449,7 @@ class ContentRouter(Transform):
         Returns:
             RouterCompressionResult with compressed content and routing metadata.
         """
+        context = context or ""
         debug_enabled = logger.isEnabledFor(logging.DEBUG)
         request_debug = (
             {

headroom/transforms/diff_compressor.py

@@ -103,7 +103,7 @@ class DiffCompressor:
         )
 
     def compress(self, content: str, context: str = "") -> DiffCompressionResult:
-        r = self._rust.compress(content, context)
+        r = self._rust.compress(content, context or "")
         cache_key: str | None = r.cache_key
         if cache_key is not None:
             # Mirror log_compressor.py + search_compressor.py: when the

tests/test_transforms/test_content_router.py

@@ -9,6 +9,7 @@ Comprehensive tests covering:
 """
 
 import json
+import logging
 from types import SimpleNamespace
 
 import pytest
@@ -472,6 +473,23 @@ class TestContentRouter:
         assert result.original == content
         assert result.strategy_used is not None
 
+    def test_compress_diff_accepts_none_context_with_debug(self, router, caplog):
+        """None context is normalized before debug logging and compressor dispatch."""
+
+        class FakeDiffCompressor:
+            def compress(self, content, context):
+                assert context == ""
+                return SimpleNamespace(compressed="diff summary")
+
+        diff = "diff --git a/file.py b/file.py\n@@ -1 +1 @@\n-old\n+new\n"
+        router._diff_compressor = FakeDiffCompressor()
+
+        caplog.set_level(logging.DEBUG, logger="headroom.transforms.content_router")
+        result = router.compress(diff, context=None)
+
+        assert result.compressed == "diff summary"
+        assert result.strategy_used == CompressionStrategy.DIFF
+
     def test_name_property(self, router):
         """Router has correct name."""
         assert router.name == "content_router"

tests/test_transforms/test_diff_compressor.py

@@ -24,6 +24,30 @@ from headroom.transforms.diff_compressor import (
 )
 
 
+def _fake_diff_result(compressed: str = "compressed") -> DiffCompressionResult:
+    return DiffCompressionResult(
+        compressed=compressed,
+        original_line_count=1,
+        compressed_line_count=1,
+        files_affected=1,
+        additions=0,
+        deletions=0,
+        hunks_kept=1,
+        hunks_removed=0,
+    )
+
+
+class _FakeRustDiffCompressor:
+    def __init__(self) -> None:
+        self.contexts: list[str] = []
+
+    def compress(self, content: str, context: str):
+        if context is None:
+            raise AssertionError("Rust diff compressor received None context")
+        self.contexts.append(context)
+        return _fake_diff_result(content)
+
+
 class TestContextReduction:
     """Tests for context line reduction."""
 
@@ -374,6 +398,32 @@ class TestEdgeCases:
         assert result.compressed is not None
 
 
+class TestContextNormalization:
+    """Tests for the Python-to-Rust diff compressor boundary."""
+
+    def test_none_and_omitted_context_become_empty_string(self) -> None:
+        compressor = object.__new__(DiffCompressor)
+        fake_rust = _FakeRustDiffCompressor()
+        compressor._rust = fake_rust
+
+        diff = "diff --git a/file.py b/file.py\n--- a/file.py\n+++ b/file.py\n"
+
+        compressor.compress(diff, context=None)
+        compressor.compress(diff)
+
+        assert fake_rust.contexts == ["", ""]
+
+    def test_non_empty_context_passes_through_unchanged(self) -> None:
+        compressor = object.__new__(DiffCompressor)
+        fake_rust = _FakeRustDiffCompressor()
+        compressor._rust = fake_rust
+
+        diff = "diff --git a/file.py b/file.py\n--- a/file.py\n+++ b/file.py\n"
+        compressor.compress(diff, context="question context")
+
+        assert fake_rust.contexts == ["question context"]
+
+
 class TestConfigOptions:
     """Tests for configuration options."""