Allow /rerun-test to checkout fork PR branch for trusted users (#21890)

.github/workflows/slash-command-handler.yml

@@ -49,14 +49,34 @@ jobs:
           fi
           echo "is_fork=$IS_FORK" >> $GITHUB_OUTPUT
           echo "ref=$(echo "$PR_DATA" | jq -r '.headRefName')" >> $GITHUB_OUTPUT
+          echo "pr_ref=refs/pull/${{ github.event.issue.number }}/head" >> $GITHUB_OUTPUT
           echo "PR owner: $HEAD_OWNER, Repo owner: $REPO_OWNER, Is fork: $IS_FORK"
 
+      - name: Check commenter permission for fork PRs
+        id: perm
+        if: steps.pr.outputs.is_fork == 'true'
+        shell: bash
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          PERM=$(gh api repos/${{ github.repository }}/collaborators/${{ github.event.comment.user.login }}/permission --jq '.permission') || {
+            PERM="none"
+            echo "::warning::Failed to check commenter permission, defaulting to none"
+          }
+          if [[ "$PERM" == "admin" || "$PERM" == "maintain" || "$PERM" == "write" ]]; then
+            echo "safe_to_checkout_pr=true" >> $GITHUB_OUTPUT
+          else
+            echo "safe_to_checkout_pr=false" >> $GITHUB_OUTPUT
+          fi
+          echo "Commenter ${{ github.event.comment.user.login }} permission: $PERM"
+
       - name: Checkout code
         uses: actions/checkout@v4
         with:
-          # For non-fork PRs, checkout PR branch to allow testing handler changes
-          # For fork PRs, stay on main for security (don't run untrusted code with elevated permissions)
-          ref: ${{ steps.pr.outputs.is_fork == 'false' && steps.pr.outputs.ref || '' }}
+          # For non-fork PRs: checkout PR branch by name
+          # For fork PRs with trusted commenter: checkout via refs/pull/N/head
+          # For fork PRs with untrusted commenter: stay on main for security
+          ref: ${{ steps.pr.outputs.is_fork == 'false' && steps.pr.outputs.ref || (steps.perm.outputs.safe_to_checkout_pr == 'true' && steps.pr.outputs.pr_ref || '') }}
 
       - name: Set up Python
         uses: actions/setup-python@v5