Restore main branch filter on push trigger to avoid duplicate CI runs

[Kludex]

The Python 3.15 support PR (#1090) accidentally dropped branches: ["main"] from the push trigger in main.yml, so every push to a PR branch triggered both a push run and a pull_request run, duplicating CI.

This restores the branch filter, and adds a concurrency block to the PR-triggered workflows so superseded runs get cancelled when new commits are pushed.

Replaces #1105, which was branched off the wrong base.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore the push branch filter

For an in-repository branch with an open PR against main or version-*, each commit still triggers both push and pull_request runs because on.push remains unfiltered. This concurrency key cannot cancel the duplicate: github.ref is the branch ref for the push run but the PR merge ref for the pull-request run, so they belong to different groups. Add the intended branches: ["main"] filter under push.

Useful? React with 👍 / 👎.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing fix-duplicate-ci-runs-2 (c83e8d2) with main (e27e2f9)}

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. ↩

[github-actions]

Docs preview: https://7835116a-httpx2-docs.pydantic.workers.dev