## Description
Extracts the event-loop callback failure classifier for a known
WebSocket disconnect regression from `server.py` into
`headroom.proxy.loop_callback_failure_policy`. The server keeps
`_is_known_websocket_callback_failure` as a compatibility alias for the
existing loop-health 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
- [x] Code refactoring (no functional changes)
## Changes Made
- Added `loop_callback_failure_policy.py` with constants for the known
message and exception shape.
- Replaced the inline server helper body with a compatibility alias to
the extracted classifier.
- Added direct classifier tests and ran the existing loop-health
regression tests.
- Carried forward the LiteLLM callback compatibility shim needed for
current mypy on `main`.
## 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_loop_callback_failure_policy.py tests\test_proxy_loop_exception_health.py
5 passed in 4.89s
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-27`.
- Exact command / steps: ran new loop callback classifier tests,
existing loop-health tests, ruff, ruff format check, mypy, and staged
gitleaks scan.
- Observed result: classifier behavior and endpoint-level loop-health
behavior remain covered; local lint/type/security checks pass.
- Not tested: live WebSocket disconnect reproduction; this slice
preserves the existing server alias.
## 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 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.
