[rejected AI] fix: respect per-request body size limits

[KayanoLiam]

Setting request.max_content_length before reading a request body now takes effect. Routes can disable the limit with None, increase it for an upload, or impose a smaller limit without changing the application-wide setting.

Check the declared Content-Length when the body is consumed instead of caching a 413 during construction. Keep the body's limit in sync with the request property, and distinguish an explicit None override from the default application setting. Streaming without Content-Length retains its existing behavior.

Fixes #425.

Validation on Python 3.13.7:

• PYTHONPATH=src .venv/bin/python -m pytest -q: 303 passed, including 48 new cases covering buffered reads, streaming, forms, and requests with and without Content-Length.
• PYTHONPATH=src .venv/bin/python -m mypy: passed.
• Ruff lint and formatting checks on the changed files: passed.

[davidism]

https://palletsprojects.com/contributing/llm-ai