chore: change to post submit and create an issue on failure

Author: sofisl

.github/workflows/librarian_generation_check.yaml

@@ -11,13 +11,19 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-name: Librarian generate diff check on pull requests
+name: Librarian generate diff check
 on:
-  pull_request:
+  push:
+    branches:
+    - main
+  workflow_dispatch:
 
 jobs:
   library_generation:
     runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+      issues: write
     steps:
     - uses: actions/checkout@v4
       with:
@@ -66,3 +72,11 @@ jobs:
     - name: Check for generated code changes
       run: |
         git diff --exit-code
+    - name: Create issue if previous step fails
+      if: ${{ failure() }}
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        gh issue create \
+          --title "Librarian generate diff check failed on main branch" \
+          --body "The librarian generate diff check failed on main branch. Please check the logs: https://github.com/googleapis/google-cloud-java/actions/runs/${GITHUB_RUN_ID}"