chore: bot validates lean-toolchain on bump/v4.X.0 branches (#20478)

.github/workflows/nightly_detect_failure.yml

@@ -170,15 +170,40 @@ jobs:
       uses: actions/github-script@v7
       with:
         script: |
-          const response = await github.rest.repos.getContent({
-            owner: context.repo.owner,
-            repo: context.repo.repo,
-            path: 'lean-toolchain',
-            ref: ${{ steps.latest_bump_branch.outputs.result }}
-          });
-          const content = Buffer.from(response.data.content, 'base64').toString();
-          const version = content.match(/leanprover\/lean4:nightly-(\d{4}-\d{2}-\d{2})/)[1];
-          return version;
+          try {
+            const response = await github.rest.repos.getContent({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              path: 'lean-toolchain',
+              ref: ${{ steps.latest_bump_branch.outputs.result }}
+            });
+            const content = Buffer.from(response.data.content, 'base64').toString();
+            const match = content.match(/leanprover\/lean4:nightly-(\d{4}-\d{2}-\d{2})/);
+            if (!match) {
+              core.setFailed('Toolchain pattern did not match');
+              core.setOutput('toolchain_content', content);
+              return null;
+            }
+            return match[1];
+          } catch (error) {
+            core.setFailed(error.message);
+            return null;
+          }
+
+    - name: Send warning message on Zulip if pattern doesn't match
+      if: failure()
+      uses: zulip/github-actions-zulip/send-message@v1
+      with:
+        api-key: ${{ secrets.ZULIP_API_KEY }}
+        email: '[email protected]'
+        organization-url: 'https://leanprover.zulipchat.com'
+        to: 'nightly-testing'
+        type: 'stream'
+        topic: 'Mathlib status updates'
+        content: |
+          ⚠️ Warning: The lean-toolchain file in the latest bump branch does not match the expected pattern 'leanprover/lean4:nightly-YYYY-MM-DD'.
+          Current content: ${{ steps.bump_version.outputs.toolchain_content }}
+          This needs to be fixed for the nightly testing process to work correctly.
 
     - name: Compare versions and post a reminder.
       env: