Expose request headers

[erikos]

Expose request headers to pipeline wrapper run methods (opt-in)

Closes #251.

run_chat_completion / run_response, and their async variants, can now receive
the incoming request headers by declaring an optional headers parameter. The
OpenAI router forwards them only when the method declares the parameter, or
**kwargs, so existing wrappers are unaffected.

This removes the need for custom middleware plus a ContextVar to get a header
such as Authorization into a wrapper, which is the usual approach today for
per-request identity or token forwarding.

Requires a fastapi-openai-compat release

Header forwarding has to happen in fastapi-openai-compat first, since that is
what invokes the run_completion callback: fastapi-openai-compat/pull/6. Until a version
containing it is installed, this change is inert. I have deliberately left the
fastapi-openai-compat>=1.2.0 floor alone here, so the pin can be bumped once
you have released the other side.

Question for maintainers

If a wrapper declares headers without a default, and the installed compat
version does not forward headers, the call fails with
TypeError: run_chat_completion() missing 1 required positional argument: 'headers',
surfacing as a 500. Two ways to handle it, and I do not want to pick for you:

1. Document it. Tell wrapper authors to write
   headers: dict[str, str] | None = None. Keeps the failure visible when the
   plumbing is wrong.
2. Pass {}. When a wrapper opts in but no headers are available, hand it an
   empty dict. The contract becomes "declare it and you always get a dict", at
   the cost of hiding a misconfiguration.

Happy to implement either.

Tests

tests/test_it_openai_headers.py deploys two wrapper fixtures through the normal
path: one declaring headers that echoes Authorization back, and one keeping
the existing signature that must be called unchanged. The end-to-end checks skip
themselves when the installed compat predates forwarding, and activate
automatically once it ships. _method_accepts_kwarg and _build_call_kwargs have
unit tests covering explicit parameters, **kwargs, wrappers that do not opt in,
unintrospectable callables, and headers=None from non-HTTP entry points.