ci: run the Tests workflow on stable-targeted PRs (unblock this promote)

Same one-liner as #772 — adds stable to tests.yml push/pull_request
filters so the required backend/frontend checks report on this PR
instead of hanging on 'Expected — Waiting for status to be reported'.

.github/workflows/tests.yml

@@ -17,9 +17,9 @@ name: Tests
 
 on:
   push:
-    branches: [main, beta]
+    branches: [main, beta, stable]
   pull_request:
-    branches: [main, beta]
+    branches: [main, beta, stable]
   workflow_dispatch:
 
 permissions: