Add CodSpeed benchmark suite

[Kludex]

Summary

• Adds a CodSpeed benchmark suite under tests/benchmarks/ covering URL parsing/building, headers, request/response construction, gzip decoding, and full Client roundtrips via MockTransport (23 benchmarks total).
• Mirrors the layout and workflow used in encode/uvicorn: pytest-based benchmarks with pytest-codspeed, run in instrumentation mode via CodSpeedHQ/action@v4.
• Wires up .github/workflows/benchmark.yml and registers the benchmark marker; coverage is configured to ignore tests/benchmarks/*.

Test plan

• CI: CodSpeed workflow runs and reports results on the PR
• uv run pytest tests/benchmarks/ passes locally (smoke)
• uv run pytest tests/benchmarks/ --codspeed runs locally without errors
• uv run ruff check tests/benchmarks/ and uv run mypy tests/benchmarks/ are clean

AI Disclaimer

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

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 7 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_bench_client_post_json: 3.9 ms
• test_bench_request_multipart: 4.5 ms
• test_bench_request_json_post: 5.9 ms
• test_bench_client_get_json: 3.5 ms
• test_bench_response_gzip_decode: 2.6 ms
• test_bench_client_get_stream: 14.8 ms
• test_bench_url_join: 8.8 ms

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[chatgpt-codex-connector]

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