BAI-3: harden live readiness diagnostics

Co-authored-by: multica-agent <github@multica.ai>

app/tests/test_job_runner.py

@@ -59,6 +59,28 @@ class JobRunnerTests(unittest.TestCase):
         release.set()
         self.assertTrue(self.runner.wait_for_idle())
 
+    def test_database_claim_rejects_the_same_job_from_another_runner(self) -> None:
+        other_runner = InProcessJobRunner(JobRegistry.load(), self.repository)
+        entered = threading.Event()
+        release = threading.Event()
+        calls = []
+
+        def wait() -> None:
+            calls.append("first")
+            entered.set()
+            release.wait(2)
+
+        self.assertTrue(self.runner.submit("screener.automatic", "shared-key", wait))
+        self.assertTrue(entered.wait(1))
+        self.assertFalse(
+            other_runner.run_inline(
+                "screener.automatic", "shared-key", lambda: calls.append("second")
+            )
+        )
+        release.set()
+        self.assertTrue(self.runner.wait_for_idle())
+        self.assertEqual(calls, ["first"])
+
     def test_failed_status_payload_is_recorded_as_failure(self) -> None:
         self.assertTrue(
             self.runner.run_inline(