Extract wire debug format policy (#1978)

## Description

Extracts opt-in Codex wire-debug formatting from `helpers.py` into
`headroom.proxy.wire_debug_format_policy`. The existing helper functions
now delegate to the pure policy so filename-safe event names and
proxy-log previews are directly testable.

Closes #

## Type of Change

- [ ] 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
- [x] Code refactoring (no functional changes)

## Changes Made

- Added `wire_debug_format_policy.py` for safe wire-debug name fragments
and compact log previews.
- Kept `_safe_event_name` and `_wire_debug_preview` in `helpers.py` as
compatibility delegates.
- Added direct tests for unsafe-name replacement, length capping, JSON
preview compaction, byte decoding/truncation, and `None` handling.
- Carried forward the LiteLLM callback compatibility shim needed for
current mypy on `main`.

## Testing

- [x] 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
python -m pytest tests\test_wire_debug_format_policy.py
5 passed in 0.19s

python -m ruff check .
All checks passed!

python -m ruff format --check .
1095 files already formatted

python -m mypy headroom --ignore-missing-imports
Success: no issues found in 409 source files

gitleaks protect --staged --no-banner --redact
no leaks found
```

## Real Behavior Proof

- Environment: Windows, Python 3.13.13, branch
`jd/architecture-slice-28`.
- Exact command / steps: ran focused wire-debug format policy tests,
ruff, ruff format check, mypy, and staged gitleaks scan.
- Observed result: formatting policy behavior is directly covered and
local lint/type/security checks pass.
- Not tested: live wire-debug capture writing; this slice preserves the
existing helper entry points.

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

## Screenshots (if applicable)

N/A

## Additional Notes

Documentation and changelog updates are N/A for this internal
architecture-only refactor. The push reported existing default-branch
Dependabot alerts; no staged secret leaks were found for this PR.

---------

Co-authored-by: Tejas Chopra <chopratejas@gmail.com>

headroom/proxy/helpers.py

@@ -24,7 +24,12 @@ from typing import TYPE_CHECKING, Any, Literal, cast
 
 from headroom import paths as _paths
 from headroom._subprocess import run
-from headroom.proxy import request_limit_policy, sse_byte_buffer_policy, wire_debug_redaction_policy
+from headroom.proxy import (
+    request_limit_policy,
+    sse_byte_buffer_policy,
+    wire_debug_format_policy,
+    wire_debug_redaction_policy,
+)
 from headroom.proxy.body_forwarding import (
     BodyMutationTracker as BodyMutationTracker,  # noqa: F401 - compatibility export
 )
@@ -119,7 +124,7 @@ def redact_for_wire_debug(value: Any) -> Any:
 
 
 def _safe_event_name(event: str) -> str:
-    return "".join(ch if ch.isalnum() or ch in ("-", "_") else "_" for ch in event)[:80]
+    return wire_debug_format_policy.safe_wire_debug_name(event)
 
 
 def _wire_debug_preview(value: Any, *, max_chars: int | None = None) -> str:
@@ -130,22 +135,7 @@ def _wire_debug_preview(value: Any, *, max_chars: int | None = None) -> str:
     deliberate trim boundary belongs.
     """
 
-    try:
-        if isinstance(value, bytes):
-            text = value.decode("utf-8", errors="replace")
-        elif isinstance(value, str):
-            text = value
-        elif value is None:
-            return ""
-        else:
-            text = json.dumps(value, ensure_ascii=False, default=str, separators=(",", ":"))
-    except Exception:
-        text = repr(value)
-
-    text = " ".join(text.split())
-    if max_chars is not None and len(text) > max_chars:
-        return text[: max_chars - 1] + "…"
-    return text
+    return wire_debug_format_policy.wire_debug_preview(value, max_chars=max_chars)
 
 
 def capture_codex_wire_debug(

headroom/proxy/wire_debug_format_policy.py

@@ -0,0 +1,35 @@
+"""Formatting policy for opt-in proxy wire debug artifacts."""
+
+from __future__ import annotations
+
+import json
+from typing import Any
+
+WIRE_DEBUG_NAME_MAX_CHARS = 80
+
+
+def safe_wire_debug_name(value: str) -> str:
+    """Return a filename-safe wire-debug name fragment."""
+    return "".join(ch if ch.isalnum() or ch in ("-", "_") else "_" for ch in value)[
+        :WIRE_DEBUG_NAME_MAX_CHARS
+    ]
+
+
+def wire_debug_preview(value: Any, *, max_chars: int | None = None) -> str:
+    """Return the compact wire payload preview used in proxy logs."""
+    try:
+        if isinstance(value, bytes):
+            text = value.decode("utf-8", errors="replace")
+        elif isinstance(value, str):
+            text = value
+        elif value is None:
+            return ""
+        else:
+            text = json.dumps(value, ensure_ascii=False, default=str, separators=(",", ":"))
+    except Exception:
+        text = repr(value)
+
+    text = " ".join(text.split())
+    if max_chars is not None and len(text) > max_chars:
+        return text[: max_chars - 1] + "…"
+    return text

tests/test_wire_debug_format_policy.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from headroom.proxy.wire_debug_format_policy import (
+    WIRE_DEBUG_NAME_MAX_CHARS,
+    safe_wire_debug_name,
+    wire_debug_preview,
+)
+
+
+def test_safe_wire_debug_name_replaces_path_unsafe_characters() -> None:
+    assert safe_wire_debug_name("req/id:with spaces") == "req_id_with_spaces"
+
+
+def test_safe_wire_debug_name_caps_length() -> None:
+    assert safe_wire_debug_name("a" * 200) == "a" * WIRE_DEBUG_NAME_MAX_CHARS
+
+
+def test_wire_debug_preview_compacts_json_like_values() -> None:
+    preview = wire_debug_preview({"message": "hello\nworld", "count": 2})
+
+    assert preview == '{"message":"hello\\nworld","count":2}'
+
+
+def test_wire_debug_preview_decodes_bytes_and_truncates() -> None:
+    assert wire_debug_preview(b"hello world", max_chars=8) == "hello w…"
+
+
+def test_wire_debug_preview_returns_empty_string_for_none() -> None:
+    assert wire_debug_preview(None) == ""