Extract tool schema savings policy

[JerrettDavis]

Description

Extracts the pure tool-schema savings attribution logic from server.py into headroom.proxy.tool_schema_savings_policy. The server keeps the _tool_schema_saved_from_tags compatibility alias used by the existing stats payload path.

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 tool_schema_savings_policy.py with stable savings tag names and pure summation behavior.
• Replaced the inline server.py helper body with a compatibility alias to the extracted policy.
• Added direct tests for valid tag summing, invalid values, non-mapping input, and stable tag names.
• Carried forward the LiteLLM callback compatibility shim needed for current mypy on main.

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_schema_savings_policy.py
4 passed in 0.14s

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-24.
• Exact command / steps: ran focused tool-schema savings policy tests, ruff, ruff format check, mypy, and staged gitleaks scan.
• Observed result: pure policy behavior is directly covered and local lint/type/security checks pass.
• Not tested: full proxy runtime; this slice only moves pure stats attribution logic while preserving the server alias.

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 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.

[github-actions]

PR governance

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