## Description
Extracts token-bucket refill, consume, wait-time, and stale-bucket
selection formulas into a pure rate-limit policy module while preserving
the async `TokenBucketRateLimiter` adapter for locks and mutable bucket
storage.
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.rate_limit_policy` for pure token-bucket
calculations.
- Updated `TokenBucketRateLimiter` to delegate refill, consume, and
stale-key selection to the extracted policy.
- Added direct tests for the rate-limit policy boundary.
- Kept the LiteLLM callback compatibility shim required for repo-wide
type checking on fresh branches.
## 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_rate_limit_policy.py tests/test_proxy_healthchecks.py tests/test_litellm_callback.py tests/test_compress_api.py::TestLiteLLMCallback -q
27 passed in 17.82s
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, local worktree based on
`headroomlabs/main`.
- Exact command / steps: ran focused rate-limit policy tests, proxy
health checks, LiteLLM callback tests, Ruff lint/format checks, mypy
over `headroom`, and staged gitleaks scan.
- Observed result: all local checks passed; staged secret scan found no
leaks.
- Not tested: full CI matrix and deployment flows; those are covered by
GitHub Actions.
## 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. GitHub reported existing Dependabot alerts on the default
branch during push; this PR does not change dependencies, and the staged
secret scan is clean.
