ci(generation-check): dynamically select fastest 3 modified packages on presubmit

[quirogas]

Summary

Updates .github/workflows/generation_check.yaml to catch unauthorized or accidental edits under packages/** during presubmit (pull_request) without relying on any static timing file or running librarian generate --all across all 245 libraries on every PR:

• Adds packages/** to dorny/paths-filter and sets fetch-depth: 2 on actions/checkout so modified files in packages/ and librarian.yaml are detected on pull_request.
• Dynamically computes candidate package weights at runtime using a MinPriorityQueue ordered by on-disk package size (totalBytes excluding node_modules/build) and API version count in librarian.yaml (score = totalBytes + apiCount * 500000).
• Runs librarian generate <pkg> on the fastest 3 modified packages during presubmit (excluding libraries marked skip_generate: true), and fails immediately if any unconfigured directory/file under packages/ is modified or if regeneration produces a dirty git working tree.
• Retains full librarian generate --all verification on push to main and workflow_dispatch.

[gemini-code-assist]

Note

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