test: cover litellm proxy hooks

[rlippmann]

What changed

• add hermetic tests for the LiteLLM proxy hook examples in tests/test_litellm_proxy_hooks.py
• cover unsupported call types, clarify blocking behavior, system-contract injection, mixed-content transcript extraction, and the replay-input versus forwarded-message boundary in the preprocessor hook
• protect the rule that preprocessor drafting may normalize replay input for compiler evaluation but must not rewrite the request messages forwarded upstream

Why

Refs #111.

This adds hermetic coverage for the LiteLLM proxy-hook behavior identified in #111. Runtime smoke/E2E coverage remains out of scope.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)