chore: change to post submit and create an issue on failure (#13422)

Converts this check to post submit and creates issue on failure. Uses
create-issue-on-failure action from librarian, so that created issue is
labeled and assigned, and subsequent failures will only add comment to
the same issue.

Verified create issue flow via temp changes
[2a20071](2a20071),
see sample issue created:
#13421.

Fixes googleapis/librarian#5240

---------

Co-authored-by: Sofia Leon <sofialeon@google.com>
Co-authored-by: sofisl <55454395+sofisl@users.noreply.github.com>

Author: zhumin8

.github/workflows/librarian_generation_check.yaml

@@ -13,11 +13,17 @@
 # limitations under the License.
 name: Librarian - Generate diff check on pull requests
 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:
@@ -70,3 +76,12 @@ jobs:
           echo "Regeneration produced code changes! Please run 'librarian generate --all' to update the generated files."
           exit 1
         fi
+    - name: Create issue if previous step fails
+      if: ${{ failure() && github.ref == 'refs/heads/main' }}
+      uses: googleapis/librarian/.github/actions/create-issue-on-failure@main
+      with:
+        title: "Librarian generate diff check failed on main branch"
+        body: |
+          The librarian generate diff check failed on main branch.
+
+          Please check the logs: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}