feat: comment on original PR after cherry-pick PR is created (#24243)

After the cherry-pick workflow creates a backport PR, it now comments on
the original PR to notify the author with a link to the new PR.

If the cherry-pick had conflicts, the comment includes a warning.

## Changes

- Capture the URL output of `gh pr create` into `NEW_PR_URL`
- Add `gh pr comment` on the original PR with the link
- Append a conflict warning to the comment when applicable

> Generated by Coder Agents

.github/workflows/cherry-pick.yaml

@@ -134,10 +134,19 @@ jobs:
             exit 0
           fi
 
-          gh pr create \
-            --base "$RELEASE_BRANCH" \
-            --head "$BACKPORT_BRANCH" \
-            --title "$TITLE" \
-            --body "$BODY" \
-            --assignee "$SENDER" \
-            --reviewer "$SENDER"
+          NEW_PR_URL=$(
+            gh pr create \
+              --base "$RELEASE_BRANCH" \
+              --head "$BACKPORT_BRANCH" \
+              --title "$TITLE" \
+              --body "$BODY" \
+              --assignee "$SENDER" \
+              --reviewer "$SENDER"
+          )
+
+          # Comment on the original PR to notify the author.
+          COMMENT="Cherry-pick PR created: ${NEW_PR_URL}"
+          if [ "$CONFLICT" = true ]; then
+            COMMENT="${COMMENT} (⚠️ conflicts need manual resolution)"
+          fi
+          gh pr comment "$PR_NUMBER" --body "$COMMENT"