chore: enable auto-merge after @NpmPackage 24h gate (CI) (#9521)

The @NpmPackage 24h gate already dismisses its block review and approves
the PR after 24 hours, but someone still has to click merge. These
bot-generated NpmPackages PRs are routine, so the merge should happen on
its own.

## Changes

After approving, the `wait-and-unblock` job now also enables auto-merge:

```sh
gh pr merge "$PR_NUMBER" --repo "$REPO" --auto --squash
```

- **Squash** because the repo disallows merge commits
(`allow_merge_commit: false`).
- Best-effort: a failure (PR already merged, or no pending required
checks) only logs a warning, so the job stays green.
- The block message now states it will also enable auto-merge.

Auto-merge is already allowed at the repo level (`allow_auto_merge:
true`), so the PR merges by itself once all required checks pass.

Follow-up to #9517

---

🤖 Generated with Claude Code

Co-authored-by: Zhe Sun <zhe@vaadin.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/npm-package-gate.yml

@@ -50,7 +50,7 @@ jobs:
 
           **No action needed — this is fully automated:**
           - I posted this block automatically when CI detected the `@NpmPackage` version bump.
-          - **24 hours after this message**, I will automatically dismiss this review and approve the PR, making it mergeable.
+          - **24 hours after this message**, I will automatically dismiss this review, approve the PR, and enable auto-merge (squash) so it merges as soon as all required checks pass.
 
           **What affects the 24h timer:**
           - New commits / pushes **do not** reset it — the countdown is anchored to this message, so it keeps running no matter how many times you push.
@@ -100,3 +100,6 @@ jobs:
           gh api -X POST "repos/$REPO/pulls/$PR_NUMBER/reviews" \
             -f event=APPROVE \
             -f body="Automatically approved 24 hours after the @NpmPackage version bump block."
+          echo "Enabling auto-merge for PR #$PR_NUMBER"
+          gh pr merge "$PR_NUMBER" --repo "$REPO" --auto --squash \
+            || echo "::warning::Could not enable auto-merge for PR #$PR_NUMBER (already merged, or no pending required checks)."