Description

Headroom already supports headroom.pipeline_extension, but request-normalization pattern was not documented. That leaves users guessing how to fix upstream quirks such as content: null tool-call payloads.

Closes #1758

Type of Change

• Documentation update

Changes Made

• Added a Pipeline Extensions section to configuration.mdx.
• Documented the PRE_SEND hook as the right place for request cleanup.
• Included a minimal NormalizeNullContent example and entry-point registration.
• Mentioned x-headroom-base-url as the per-request routing override.

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

Docs-only verification:
- Reviewed docs diff for API names, hook names, and placement.
- Confirmed example uses public `headroom.pipeline` contract and matches existing header-routing terminology.

Real Behavior Proof

• Environment: GitHub PR diff review for docs-only extension recipe.
• Exact command / steps: Compared new configuration docs against public pipeline-extension and per-request routing interfaces already exposed by Headroom.
• Observed result: Docs now show concrete request-cleanup extension pattern without requiring a fork.
• Not tested: Live extension package execution in this verification pass.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable