fix(test): raise spawn-child join timeout in test_multi_process_visibility (#13588)

The test spawns a child via multiprocessing spawn context, which
cold-imports the full langflow package (plus coverage's multiprocessing
hooks in CI) before appending a single event. The 10s join timeout is
routinely exceeded on a loaded CI runner sharing 4 vCPUs with a second
xdist worker: in nightly run 27253229568 (Unit Tests - Python 3.12 -
Group 5) the test failed all 12 executions (5 reruns x 2 step attempts),
each rerun exactly 10s apart - the join deadline, not a product bug.

Raise the liveness bound to 60s (join returns immediately when the
child exits, so the passing case is unaffected) and kill the child on
timeout so a hung spawn can't leak into later tests.

Author: erichare

src/backend/tests/unit/test_flow_events_service.py

@@ -225,7 +225,13 @@ def test_multi_process_visibility(tmp_path):
         args=(str(shared_dir), "flow-mp", "component_added", "from-other-process"),
     )
     proc.start()
-    proc.join(timeout=10)
+    # Generous bound: a spawned child cold-imports the full langflow package
+    # (plus coverage's multiprocessing hooks in CI), which can take >10s on a
+    # loaded CI runner sharing 4 vCPUs with another xdist worker.
+    proc.join(timeout=60)
+    if proc.is_alive():
+        proc.kill()
+        proc.join(timeout=5)
     assert proc.exitcode == 0, "Child process failed to append event"
 
     reader = FlowEventsService(cache_dir=shared_dir)