fix(ng-dev): handle the 409 GitHub returns when a validated PR head moved (#3911)

Pinning the API merge to the validated head SHA means GitHub now rejects
the merge with a 409 if the head moved after validation, but that error
was falling through to the generic `throw e` and surfacing as a raw
Octokit exception instead of a clear, actionable message.

Adds a branch alongside the existing 403/404 handling that catches the
409 and raises a FatalMergeToolError explaining what happened (the head
changed after validation) and what to do (re-run the merge).

PR Close #3911

Author: herdiyana256

.github/local-actions/branch-manager/main.js

@@ -144,6 +144,16 @@ export class GithubApiMergeStrategy extends AutosquashMergeStrategy {
       if (isGithubApiError(e) && (e.status === 403 || e.status === 404)) {
         throw new FatalMergeToolError('Insufficient Github API permissions to merge pull request.');
       }
+      // Github returns `409` when the `sha` we pinned the merge to no longer matches the pull
+      // request's actual head. That means the pull request's head moved after it was validated
+      // (approvals, CI status) but before this merge call went out.
+      if (isGithubApiError(e) && e.status === 409) {
+        throw new FatalMergeToolError(
+          `Pull request head commit changed after it was validated (expected ${headSha}). ` +
+            `Merging now would land commits that were never reviewed or checked. Please re-run ` +
+            `the merge so the new head is validated.`,
+        );
+      }
       throw e;
     }