refactor(proxy): extract tool injection policy (#1995)

## Description

Extracts memory tool injection stickiness configuration parsing from
`headroom.proxy.helpers` into a focused policy module. The existing
helper functions remain in place for the session tool tracker and sticky
injection helpers.

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 `headroom.proxy.tool_injection_policy` for tool sticky mode and
tracker session limit resolution.
- Kept `get_tool_injection_sticky_mode()` and
`get_tool_tracker_max_sessions()` as compatibility wrappers in
`helpers.py`.
- Added direct unit tests for defaults, accepted values, and loud
rejection of invalid operator configuration.

## 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_tool_injection_policy.py tests/test_memory_tool_session_sticky.py
38 passed in 0.44s

python -m ruff check .
All checks passed!

python -m ruff format --check .
1069 files already formatted

python -m mypy headroom --ignore-missing-imports
Success: no issues found in 410 source files

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

## Real Behavior Proof

- Environment: Windows, Python 3.13.13
- Exact command / steps: Ran focused tool injection policy tests,
existing memory tool sticky suite, full ruff, format check, mypy, and
staged gitleaks scan.
- Observed result: Existing sticky memory-tool behavior remains green
while extracted policy parsing is covered directly.
- Not tested: Full repository pytest suite locally; CI covers the
broader matrix.

## 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 not applicable for this internal
refactor. The default-branch Dependabot alerts reported during push are
pre-existing and unrelated to this PR.

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

headroom/proxy/handlers/anthropic.py

@@ -3434,7 +3434,10 @@ class AnthropicHandlerMixin:
                     # blocks every other request for the duration; a timeout
                     # here is caught below and passes the item through.
                     result = await self._run_compression_in_executor(
-                        lambda messages=messages, model=model, context_limit=context_limit, frozen_message_count=frozen_message_count: (
+                        lambda messages=messages,
+                        model=model,
+                        context_limit=context_limit,
+                        frozen_message_count=frozen_message_count: (
                             self.anthropic_pipeline.apply(
                                 messages=messages,
                                 model=model,

headroom/proxy/tool_injection_config.py

@@ -3,41 +3,35 @@
 from __future__ import annotations
 
 import os
-from typing import Literal, cast
 
-TOOL_INJECTION_STICKY_ENV = "HEADROOM_TOOL_INJECTION_STICKY"
-ToolInjectionStickyMode = Literal["enabled", "disabled"]
-TOOL_INJECTION_STICKY_DEFAULT: ToolInjectionStickyMode = "enabled"
+from headroom.proxy.tool_injection_policy import (
+    TOOL_INJECTION_STICKY_DEFAULT,
+    TOOL_INJECTION_STICKY_ENV,
+    TOOL_TRACKER_MAX_SESSIONS_DEFAULT,
+    TOOL_TRACKER_MAX_SESSIONS_ENV,
+    ToolInjectionStickyMode,
+    resolve_tool_injection_sticky_mode,
+    resolve_tool_tracker_max_sessions,
+)
 
-TOOL_TRACKER_MAX_SESSIONS_ENV = "HEADROOM_TOOL_TRACKER_MAX_SESSIONS"
-TOOL_TRACKER_MAX_SESSIONS_DEFAULT = 1000
+__all__ = [
+    "TOOL_INJECTION_STICKY_DEFAULT",
+    "TOOL_INJECTION_STICKY_ENV",
+    "TOOL_TRACKER_MAX_SESSIONS_DEFAULT",
+    "TOOL_TRACKER_MAX_SESSIONS_ENV",
+    "ToolInjectionStickyMode",
+    "get_tool_injection_sticky_mode",
+    "get_tool_tracker_max_sessions",
+]
 
 
 def get_tool_injection_sticky_mode() -> ToolInjectionStickyMode:
     """Return the active memory-tool stickiness mode."""
 
-    raw = os.environ.get(TOOL_INJECTION_STICKY_ENV, "").strip().lower()
-    if not raw:
-        return TOOL_INJECTION_STICKY_DEFAULT
-    if raw in ("enabled", "disabled"):
-        return cast(ToolInjectionStickyMode, raw)
-    raise ValueError(
-        f"Invalid {TOOL_INJECTION_STICKY_ENV}={raw!r}; expected 'enabled' or 'disabled'"
-    )
+    return resolve_tool_injection_sticky_mode(os.environ.get(TOOL_INJECTION_STICKY_ENV))
 
 
 def get_tool_tracker_max_sessions() -> int:
     """Return the LRU bound for memory tool session tracking."""
 
-    raw = os.environ.get(TOOL_TRACKER_MAX_SESSIONS_ENV, "").strip()
-    if not raw:
-        return TOOL_TRACKER_MAX_SESSIONS_DEFAULT
-    try:
-        value = int(raw)
-    except ValueError as exc:
-        raise ValueError(
-            f"Invalid {TOOL_TRACKER_MAX_SESSIONS_ENV}={raw!r}; expected positive int"
-        ) from exc
-    if value <= 0:
-        raise ValueError(f"Invalid {TOOL_TRACKER_MAX_SESSIONS_ENV}={raw!r}; expected positive int")
-    return value
+    return resolve_tool_tracker_max_sessions(os.environ.get(TOOL_TRACKER_MAX_SESSIONS_ENV))

headroom/proxy/tool_injection_policy.py

@@ -0,0 +1,44 @@
+"""Policy helpers for memory tool injection stickiness configuration."""
+
+from __future__ import annotations
+
+from typing import Literal, cast
+
+TOOL_INJECTION_STICKY_ENV = "HEADROOM_TOOL_INJECTION_STICKY"
+ToolInjectionStickyMode = Literal["enabled", "disabled"]
+TOOL_INJECTION_STICKY_DEFAULT: ToolInjectionStickyMode = "enabled"
+
+TOOL_TRACKER_MAX_SESSIONS_ENV = "HEADROOM_TOOL_TRACKER_MAX_SESSIONS"
+TOOL_TRACKER_MAX_SESSIONS_DEFAULT = 1000
+
+
+def resolve_tool_injection_sticky_mode(raw: str | None) -> ToolInjectionStickyMode:
+    """Resolve memory-tool injection stickiness mode from an environment value."""
+
+    normalized = (raw or "").strip().lower()
+    if not normalized:
+        return TOOL_INJECTION_STICKY_DEFAULT
+    if normalized in ("enabled", "disabled"):
+        return cast(ToolInjectionStickyMode, normalized)
+    raise ValueError(
+        f"Invalid {TOOL_INJECTION_STICKY_ENV}={normalized!r}; expected 'enabled' or 'disabled'"
+    )
+
+
+def resolve_tool_tracker_max_sessions(raw: str | None) -> int:
+    """Resolve the positive LRU session bound for tool injection tracking."""
+
+    normalized = (raw or "").strip()
+    if not normalized:
+        return TOOL_TRACKER_MAX_SESSIONS_DEFAULT
+    try:
+        value = int(normalized)
+    except ValueError as exc:
+        raise ValueError(
+            f"Invalid {TOOL_TRACKER_MAX_SESSIONS_ENV}={normalized!r}; expected positive int"
+        ) from exc
+    if value <= 0:
+        raise ValueError(
+            f"Invalid {TOOL_TRACKER_MAX_SESSIONS_ENV}={normalized!r}; expected positive int"
+        )
+    return value

tests/test_tool_injection_policy.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import pytest
+
+from headroom.proxy.tool_injection_policy import (
+    TOOL_INJECTION_STICKY_ENV,
+    TOOL_TRACKER_MAX_SESSIONS_DEFAULT,
+    TOOL_TRACKER_MAX_SESSIONS_ENV,
+    resolve_tool_injection_sticky_mode,
+    resolve_tool_tracker_max_sessions,
+)
+
+
+def test_resolve_tool_injection_sticky_mode_defaults_to_enabled() -> None:
+    assert resolve_tool_injection_sticky_mode(None) == "enabled"
+    assert resolve_tool_injection_sticky_mode("  ") == "enabled"
+
+
+def test_resolve_tool_injection_sticky_mode_accepts_known_values() -> None:
+    assert resolve_tool_injection_sticky_mode("ENABLED") == "enabled"
+    assert resolve_tool_injection_sticky_mode(" disabled ") == "disabled"
+
+
+def test_resolve_tool_injection_sticky_mode_rejects_unknown_values() -> None:
+    with pytest.raises(ValueError, match=TOOL_INJECTION_STICKY_ENV):
+        resolve_tool_injection_sticky_mode("maybe")
+
+
+def test_resolve_tool_tracker_max_sessions_defaults() -> None:
+    assert resolve_tool_tracker_max_sessions(None) == TOOL_TRACKER_MAX_SESSIONS_DEFAULT
+    assert resolve_tool_tracker_max_sessions("  ") == TOOL_TRACKER_MAX_SESSIONS_DEFAULT
+
+
+def test_resolve_tool_tracker_max_sessions_accepts_positive_int() -> None:
+    assert resolve_tool_tracker_max_sessions("42") == 42
+    assert resolve_tool_tracker_max_sessions(" 7 ") == 7
+
+
+@pytest.mark.parametrize("raw", ["0", "-1", "not-int", "1.5"])
+def test_resolve_tool_tracker_max_sessions_rejects_invalid_values(raw: str) -> None:
+    with pytest.raises(ValueError, match=TOOL_TRACKER_MAX_SESSIONS_ENV):
+        resolve_tool_tracker_max_sessions(raw)