## Description
Extracts proxy mode normalization into a pure `proxy_mode_policy`
module. `modes.py` keeps the existing public API and logging, while
alias/default/unknown-mode decisions are now represented by a
deterministic value object with direct tests.
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.proxy_mode_policy` with canonical mode
constants, alias mapping, `ProxyModeDecision`, and pure normalization
helpers.
- Updated `headroom.proxy.modes` to delegate normalization decisions
while preserving existing constants, predicates, fallback behavior, and
logging.
- Added direct policy tests for canonical modes, legacy aliases, blank
values, unknown values, and value-only normalization.
- Included the current LiteLLM callback signature compatibility shim
required for repo-wide mypy on main-based slices.
## 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_proxy_mode_policy.py tests/test_proxy_modes.py tests/test_litellm_callback.py -q
17 passed in 7.84s
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, clean worktree based on
`headroomlabs/main`.
- Exact command / steps: targeted pytest, ruff, format check, repo-wide
mypy, staged gitleaks scan.
- Observed result: proxy mode policy/modes/callback tests pass; static
checks pass; no staged secrets detected.
- Not tested: live proxy run; this slice preserves existing public mode
helpers and only moves pure normalization policy.
## 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
architecture slice. PR-specific GHAS checks will be monitored after
opening.
