## Description
`litellm` is a hard dependency and its metadata caps `Requires-Python
>=3.10,<3.14`, so `pip install headroom-ai` is unsatisfiable on Python
3.14. But litellm is only used for model registry / pricing / non-core
providers — all lazily imported behind `ImportError` guards — never on
the core compression or Anthropic proxy path. Refs #956 (install half).
## Type of Change
- [x] Bug fix (non-breaking change that fixes an issue)
## Changes Made
- Add a `python_version < '3.14'` marker to both litellm declarations
(core deps + dev extra); installs unchanged on <=3.13, skipped on 3.14
(matches the existing rapidocr/tomli marker pattern).
## Testing
- [x] Unit tests pass (`pytest`)
- [x] New tests added for new functionality
- [x] Manual testing performed
### Test Output
```text
$ pytest tests/test_litellm_optional.py -q
2 passed in 0.10s
$ python3.14 -m pip install dist/headroom_ai-0.25.0-cp310-abi3-linux_x86_64.whl
Successfully installed headroom-ai-0.25.0 ... # litellm NOT installed
$ python3.14 -c "import importlib.util as u; print(u.find_spec('litellm') is not None)"
False
```
## Real Behavior Proof
- Environment: fresh venv on CPython 3.14.5, Linux
- Exact command / steps: built the abi3 wheel, `pip install` it on
Python 3.14, then `import headroom` + start the proxy + send a
compressible request
- Observed result: install exits 0 with litellm skipped; `import
headroom` works; the proxy compresses (29913 -> 27626 tokens). Stock
0.25.0 cannot install on 3.14 at all.
- Not tested: litellm-backed features on 3.14 (intentionally unavailable
there until litellm supports 3.14)
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
