fix: strip x-openai-internal-* headers when proxying to upstreams

[GrabbyGrabby]

Description

This PR prevents upstreams from rejecting proxied requests when internal headers like X-OpenAI-Internal-Codex-Responses-Lite are forwarded by the client. These headers are stripped case-insensitively before proxying.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Added strip_openai_internal_headers(headers) utility function to case-insensitively pop all x-openai-internal-* headers.
• Integrated the stripping utility into the main proxy pathways in headroom/proxy/handlers/openai.py:
  • openai_chat_completions (chat path)
  • openai_completions (legacy text path)
  • v1_responses (passthrough path)
• Created the corresponding unit test test_strip_openai_internal_headers in tests/test_proxy_handler_helpers.py to assert header scrubbing correctness.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

pytest tests/test_proxy_handler_helpers.py
...
✔ test_strip_openai_internal_headers passes (1.02ms)

[github-actions]

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