Skip cookie extraction without Set-Cookie

[Kludex]

Summary

Skip CookieJar.extract_cookies() when a response has no Set-Cookie header.

The current path constructs urllib request/response compatibility wrappers and an email message for every response, even though most responses cannot update the cookie jar. Checking the already-parsed response headers first avoids that work while preserving cookie handling when Set-Cookie is present.

Related to #827.

Benchmark

The #827 localhost benchmark uses 100 requests at concurrency 20 after a 100-request warmup. Median across three 10-round runs:

	main	This PR
Total time	42.14 ms	38.75 ms

This is an approximately 8% improvement for HTTPX2 in this workload.

Validation

• Added a regression test proving that the cookie jar is not invoked without Set-Cookie.
• Existing multiple-Set-Cookie coverage continues to pass.
• Full suite: 1,909 passed, 1 skipped.
• Mypy, Ruff, formatting, and git diff --check pass.

[github-actions]

Docs preview: https://fbc99cb7-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing perf/cookie-extraction-fast-path (ebb0f94) with main (89c644d)}

Footnotes

1. 7 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. ↩

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 5d3dfd3464

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[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>