## Description
Extracts deterministic beta-header token parsing and merge rules from
`headroom.proxy.helpers` into a focused module. Existing helper names
remain available for Anthropic/OpenAI handlers and the session beta
tracker.
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.beta_header_merge` for beta token splitting and
deterministic merge behavior.
- Re-exported the existing `merge_anthropic_beta` and
`merge_openai_beta` helper names from `helpers.py` for compatibility.
- Added direct unit tests for token splitting, ordering,
case-insensitive dedupe, empty required tokens, and provider wrappers.
## 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_beta_header_merge.py tests/test_anthropic_beta_session_sticky.py tests/test_openai_beta_session_sticky.py
48 passed in 0.38s
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 beta merge tests, existing
Anthropic/OpenAI beta sticky suites, full ruff, format check, mypy, and
staged gitleaks scan.
- Observed result: Existing beta merge and tracker behavior remains
green while extracted merge rules are 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.
