Add SSE chunking benchmark

[Kludex]

Summary

• Add a benchmark for SSE events split across many chunks before a line separator.
• Exercise the public EventSource API so the benchmark reflects user-facing behavior.

Motivation

Track SSE parsing performance for highly fragmented event streams.

Validation

• uv run pytest tests/test_benchmark.py::test_bench_sse_many_chunks_without_line_separator -q
• Ruff formatting and lint checks
• mypy

[github-actions]

Docs preview: https://653abfad-httpx2-docs.pydantic.workers.dev

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

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[codspeed-hq]

Merging this PR will not alter performance

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	test_bench_sse_many_chunks_without_line_separator	N/A	3.1 s	N/A

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_{Comparing worktree-rippling-bouncing-hare (d2c0051) with main (e0b0124)}

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. ↩