Support additional session request headers

[bc-anam]

Summary

• add optional ClientOptions.request_headers for preview/proxy HTTP headers
• ensure SDK-owned authorization and content-type headers cannot be overridden
• cover defaults and header precedence in tests

Verification

• uv run ruff format --check .
• uv run ruff check .
• uv run pytest -q (71 passed)

uv run mypy src still reports the existing aiortc/av typing errors also present on main.

---

Summary by cubic

Adds support for extra HTTP headers on session start via ClientOptions.request_headers. SDK-owned Authorization and Content-Type remain enforced and cannot be overridden.

• New Features

  • Merges request_headers into session request headers while preserving SDK values for Authorization and Content-Type.
  • Tests cover default None and header precedence (e.g., x-vercel-protection-bypass).

• Dependencies

  • Bump anam in uv.lock to 0.9.0a2.

^{Written for commit 6514741. Summary will update on new commits.}

[anam-push-bot]

Auto-approved with label(s): low-risk