refactor(proxy): extract tool injection tracker

[JerrettDavis]

Description

Extracts the sticky memory tool session tracker from headroom.proxy.helpers into a focused state module. helpers.SessionToolTracker remains as an env-aware compatibility wrapper so existing injection and singleton call sites keep the same API.

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
• Code refactoring (no functional changes)

Changes Made

• Added headroom.proxy.tool_injection_tracker.SessionToolTracker as the pure bounded LRU state holder.
• Replaced the large in-helper tracker implementation with a small env-aware wrapper.
• Added direct tracker tests for unknown sessions, ordered golden bytes, first-write wins, provider isolation, LRU eviction, and input validation.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

python -m pytest tests/test_tool_injection_tracker.py tests/test_memory_tool_session_sticky.py tests/test_corrupt_golden_bytes_recovery.py
44 passed in 0.54s

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 direct tracker tests, sticky memory tool tests, corrupt golden byte recovery tests, full ruff, format check, mypy, and staged gitleaks scan.
• Observed result: Existing sticky injection behavior and recovery behavior remain green while the tracker state domain is directly covered.
• Not tested: Full repository pytest suite locally; CI covers the broader matrix.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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
• 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.

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.