Avoid quadratic copying when sending large HTTP/2 request bodies

[Kludex]

Closes #1039.

_send_stream_data rebuilt the remaining body tail as a fresh bytes object on every frame, copying ~n²/2k bytes for an n byte body sent in k byte frames. This replaces it with an offset-based loop so each byte is sliced once.

Benchmark sending a single bytes body through HTTP2Connection over a MockStream (16 KiB frames):

Body size	Before	After
16 MiB	0.153s	0.005s
64 MiB	2.226s	0.021s
256 MiB	35.748s	0.088s

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://2c961427-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will improve performance by ×290

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 16 untouched benchmarks
⏩ 7 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	test_bench_http2_send_large_body	16,375.9 KB	55.9 KB	×290

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing http2-offset-send (e891529) with main (d6d7ec3)}

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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[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}