Pick the run this test made, not the first one running

The suite runs in a random order and a run left marked running by an
earlier module lands in the same list, so asserting on data[0] failed on
whichever seed put that module first.

backend/tests/api/routes/test_observability.py

@@ -278,7 +278,10 @@ def test_a_running_cascade_reports_how_long_it_has_been_going(
         f"{PREFIX}/runs", headers=superuser_token_headers, params={"status": "running"}
     ).json()
 
-    assert runs["data"][0]["duration_ms"] >= 30_000
+    # By id rather than by position: another module can leave a run of its own
+    # still marked running, and the list is not this test's alone.
+    in_flight = next(r for r in runs["data"] if r["id"] == "in-flight")
+    assert in_flight["duration_ms"] >= 30_000
 
 
 def test_events_narrow_to_one_minute(