fix(agent): fix race condition in test_server backup terminal write test

The test polls /ping until 503 (set when _background_pipeline_failed
becomes True) then immediately asserts write_terminal was called. But
write_terminal runs after the flag is set in the same except block,
creating a race. Fix by joining background threads before asserting.

Author: bgagent

agent/tests/test_server.py

@@ -63,6 +63,13 @@ def boom(**_kwargs):
     assert body["status"] == "unhealthy"
     assert body["reason"] == "background_pipeline_failed"
 
+    # Wait for the background thread to finish so write_terminal has been called
+    # (the 503 flag is set before write_terminal in the except block)
+    with server._threads_lock:
+        threads = list(server._active_threads)
+    for t in threads:
+        t.join(timeout=5)
+
     mock_write.assert_called()
     call_kw = mock_write.call_args
     assert call_kw[0][0] == "task-crash-1"