Run HTTP compliance checks in CI

[Kludex]

Summary

• add a dedicated HTTP Compliance workflow for the multipart form-data receiver profile
• add the subprocess adapter used to exercise python-multipart's public parser API
• pin the runner revision and Rust toolchain

Known preamble and transport-padding behavior remains excluded.

Testing

• 34 HTTP Compliance executions passed
• Ruff
• mypy
• check-sdist
• Zizmor

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks
⏩ 5 skipped benchmarks¹

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_{Comparing add-http-compliance-ci (6a6f0a6) with main (d9cb4c6)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>