[ML] Improve backport automation with label copying and pending tracking

[edsavage]

Summary

Improves the backport automation with three changes based on feedback from PR #2917:

1. copySourcePRLabels (.backportrc.json): Copies labels from the original PR to backport PRs using a regex that excludes backport and v* version labels (to avoid recursive backport triggers). Labels like >bug, >enhancement, :ml will now appear on backport PRs automatically.

2. sourcePRLabels (.backportrc.json): Adds a backport-pending label to the original PR when backport PRs are created, making it easy to filter which PRs still need backport attention.

3. Automatic backport-pending removal (backport.yml): When a backport PR is merged, a new remove-backport-pending job extracts the original PR number from the title, checks if all sibling backport PRs are also merged/closed, and removes the backport-pending label from the original PR if none remain open. This completes the full lifecycle — no manual label removal needed.

How it works

Original PR merged with v9.2.7 + v9.3.2 labels
    |
    |-> Backport tool creates backport PRs for 9.2 and 9.3
    |   |- Copies >bug, :ml labels to backport PRs (copySourcePRLabels)
    |   +- Adds backport-pending to original PR (sourcePRLabels)
    |
    |-> Backport PR for 9.3 merged
    |   +- remove-backport-pending job runs -> 1 open backport remains -> label kept
    |
    +-> Backport PR for 9.2 merged
        +- remove-backport-pending job runs -> 0 open backports -> label removed

Test plan

• Merge a PR with version labels and >bug/:ml labels
• Verify backport PRs are created with >bug and :ml labels copied
• Verify the original PR gets the backport-pending label added
• Verify backport and v* labels are NOT copied to backport PRs
• Merge one backport PR -> verify backport-pending remains on the original PR
• Merge the remaining backport PR(s) -> verify backport-pending is removed from the original PR

[prodsecmachine]

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