Fix copy_metadata execution message to reflect what was actually copied

AmR Mahdyrequested to merge
gitlab-community/gitlab-org/gitlab:fix-copy-metadata-message-510184 into master

What does this MR do and why?

Fixes the execution message for the /copy_metadata quick action so it accurately reflects what was actually copied from the source issuable, instead of always saying "Copied labels and milestone from..." regardless of the source content.

Before: Always showed Copied labels and milestone from #123.

After: Shows a message matching what was actually present on the source:

  • Both labels and milestone → Copied labels and milestone from #123.
  • Labels only → Copied labels from #123.
  • Milestone only → Copied milestone from #123.
  • Neither → No metadata to copy from #123.

The core bug (milestone not being copied at all) was already fixed in a prior commit. This MR addresses the remaining cleanup item from the issue.

Closes #510184

MR acceptance checklist

  • This MR does not introduce a new feature flag
  • Tests added for all new message variants (labels only, milestone only, both, neither)
  • No unrelated changes

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