Fix changelog extraction regex for H2 release headings

[Kludex]

The changelog now uses H2 headings for releases (## 2.10.0), but the hatch-fancy-pypi-readme fragment pattern in src/httpx2/pyproject.toml still expected the extracted release section to start with an H3 heading (\n(###.+?\n)## ). As a result, the generated PyPI README's Release Information section started at the first subsection (e.g. ### Added) and omitted the release heading and any leading bullets.

This updates the pattern to \n(## [0-9].+?)(?=\n## |$), which skips the Unreleased section, captures the complete latest release section, and stops at the next release heading (or end of file when only one release exists).

Validated by building the httpx2 wheel and confirming the metadata now contains the full 2.10.0 section while excluding Unreleased and 2.9.1.

[github-actions]

Docs preview: https://9a950bfc-httpx2-docs.pydantic.workers.dev

[chatgpt-codex-connector]

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[codspeed]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing fix-changelog-readme-extraction (4d21ac9) with main (6f38ddf)}

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}