Refactor SSE parser coordination

[Kludex]

Summary

• Rename the line-to-event decoder to _SSEEventDecoder.
• Introduce _SSEParser to own line framing, event decoding, and pending-size coordination.
• Simplify the synchronous and asynchronous EventSource loops to consume a single parser abstraction.

Motivation

Keep SSE parsing responsibilities encapsulated and remove duplicated orchestration from EventSource. This is a behavior-preserving internal refactor with no public API changes.

Validation

• pytest tests/httpx2/test_sse.py -q
• Ruff formatting and lint checks
• mypy
• git diff --check

[github-actions]

Docs preview: https://3ae1e58e-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing agent/refactor-sse-parser (119476b) with main (9849196)}

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. ↩

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}