ci: harden pull request label workflow

[leoafarias]

Related issue

Not tied to a product issue.

Description

Make the pull-request label workflow independent of a local checkout and avoid
the issue-label lookup that failed with a transient stale-node HTTP 422 after
the repository move.

Changes

• Read changed PR files through the paginated Pull Requests API.
• Reuse labels already present in the pull_request_target event payload.
• Remove the unnecessary checkout of the untrusted PR head and the unused bot
  token environment variable.
• Upgrade actions/github-script to the Node 24-based v9 release and retry
  transient API failures.

Review Checklist

• Testing: YAML parsing, the changed-files REST request, and a fresh
  pull_request_target run on feat(mix_generator): emit styler field metadata #1028 were verified.
• Breaking Changes: No. Label selection and removal behavior is unchanged.
• Documentation Updates: Not required for an internal workflow repair.
• Website Updates: Not required.

Additional Information (optional)

The failure occurred before label calculation completed because
listLabelsOnIssue returned HTTP 422 for PR #1027. The event payload already
contains the same label snapshot, so that request was redundant.

The workflow intentionally preserves its existing ownership of the complete PR
label set: labels not derived from changed package paths (or repo) are
removed.