## Description
Extracts mixed-content parsing out of the large `ContentRouter` module
into a pure transform-domain module. The router still exports the
existing compatibility names, but section typing, mixed-content
indicators, section splitting, and JSON block extraction now live in a
focused domain object/function layer.
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.transforms.mixed_content` with `ContentSection`,
`mixed_content_indicators`, `is_mixed_content`, `split_into_sections`,
and JSON block extraction.
- Updated `ContentRouter` to delegate mixed-content debug indicators and
parsing to the new module while preserving legacy imports from
`content_router.py`.
- Added direct unit coverage for mixed-content detection, section
boundaries, and JSON delimiters inside string literals.
- Included the LiteLLM callback signature compatibility shim needed for
repo-wide mypy while the earlier architecture PRs are still open.
## 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_mixed_content_sections.py tests/test_transforms_content_router.py tests/test_litellm_callback.py tests/test_compress_api.py::TestLiteLLMCallback -q
50 passed in 6.82s
python -m ruff check .
All checks passed!
python -m ruff format --check .
1095 files already formatted
python -m mypy headroom --ignore-missing-imports
headroom\proxy\server.py:1457: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs [annotation-unchecked]
headroom\proxy\server.py:1468: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs [annotation-unchecked]
Success: no issues found in 409 source files
```
## Real Behavior Proof
- Environment: Windows, Python 3.13.13, clean worktree
`C:\git\headroom-pr-slice6`
- Exact command / steps: ran the pytest, Ruff, format, and mypy commands
listed above.
- Observed result: mixed-content parsing behavior remains covered
through existing router tests and new direct tests; repo-wide lint/type
checks pass.
- Not tested: full pytest suite and Docker/native CI jobs are left to
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
- [ ] 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, changelog, and screenshots are N/A for this internal
refactor.
- Manual UI testing is N/A; this is pure transform parsing logic.
- Comment checklist is unchecked because the extracted functions are
small and covered by direct tests.
