Add HTTP/2 large-body send benchmark and CodSpeed memory mode

[Kludex]

Adds a benchmark exercising the HTTP/2 request-body send path (HTTP2Connection sending an 8 MiB bytes body over a MockStream with a large flow window), and a second CodSpeed job running the suite in memory mode.

This establishes a baseline so the CPU and allocation improvements in #1127 show up on CodSpeed - the send path on main currently does quadratic tail copying (#1039).

AI Disclaimer

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

[github-actions]

Docs preview: https://4058bca7-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks
🆕 1 new benchmark
⏩ 7 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Memory	test_bench_http2_send_large_body	N/A	16 MB	N/A

---

_{Comparing http2-memory-benchmark (729b5ab) with main (dec24ad)}

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]

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[chatgpt-codex-connector]

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