Notify GChat on CI workflow failure

Summary:
Adds a separate `notify` job to the OSS CI workflow that POSTs a Google
Chat message (URL stored in `GCHAT_WEBHOOK_URL` repo secret) when the
`build-and-test` matrix has any failing leg. The message includes repo,
branch, workflow name, and a direct link to the failed run.

Why a workflow-level `notify` job (not per-step or per-leg):
- The `build-and-test` matrix has 6 legs (2 OS x 3 Python). A per-step
  `if: failure()` notification would send up to 6 GChat messages on a
  single broken commit. Aggregating into a downstream `notify` job
  collapses that to a single notification per workflow run.
- `needs: build-and-test` makes the notify job run after every leg
  finishes; `if: failure()` at job scope evaluates the rolled-up status
  of needed jobs, so it fires exactly once when any leg failed (and
  not at all on success or cancellation).
- Keeps the `build-and-test` job focused on actual build/test work; the
  notify side-effect is composed in via the workflow DAG.

Reviewed By: pdepetro

Differential Revision: D104847779

fbshipit-source-id: 6cdafde56c85ffdb88f3a7ce54b65100c8fa6147

Author: bvu405

.github/workflows/ci.yml

@@ -54,3 +54,28 @@ jobs:
 
       - name: Run tests
         run: pytest tests/ -v
+
+  notify:
+    name: notify-on-failure
+    needs: build-and-test
+    # Single workflow-level notification: fires once if ANY leg of the
+    # build-and-test matrix failed, instead of per-leg (avoids 6x noise
+    # from the 2 OS x 3 Python matrix). `failure()` at job scope returns
+    # true when any job in `needs:` has the `failure` result.
+    if: ${{ failure() }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Notify GChat on failure
+        env:
+          GCHAT_WEBHOOK_URL: ${{ secrets.GCHAT_WEBHOOK_URL }}
+          REPO: ${{ github.repository }}
+          REF_NAME: ${{ github.ref_name }}
+          WORKFLOW: ${{ github.workflow }}
+          RUN_URL: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}
+        run: |
+          text=$(printf '❌ *GitHub Action Failed*\n\n*Repo:* %s\n*Branch:* %s\n*Workflow:* %s\n*Run:* %s' \
+            "$REPO" "$REF_NAME" "$WORKFLOW" "$RUN_URL")
+          curl -X POST "$GCHAT_WEBHOOK_URL" \
+            -H 'Content-Type: application/json' \
+            -d "$(jq -n --arg text "$text" '{text: $text}')"