fix(web): a cancelled on-demand start never reaches the display after its replacement

CI caught a race in test_a_new_start_supersedes_the_pending_one: the
dispatcher's worker could read the old start, the route cancel it and send
the new one, and the worker's send of the old one land after it. cancel()
now waits out a send already in flight (the worker holds a send lock while
it reads the pending start and sends it), so whatever the caller sends next
lands after it. Pinned by test_a_cancel_waits_for_a_send_in_flight, which
fails without the wait.

The Linux-only TestRealSocket test for a display that went away now
expects the 202 and the start-timeout that follows, as the route answers
since the background dispatcher.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_api_v3_on_demand_socket.py

@@ -306,8 +306,11 @@ class TestNoDisplayListening:
             resp = api_v3_client.post(START_URL, json={"plugin_id": "clock"})
             assert resp.status_code == 200
             outcome = self._outcome()
-        assert sent == [resp.get_json()["data"]["request_id"]]
-        assert old not in sent
+        # The old start is cancelled before the new one is sent, and
+        # cancel() waits out a send of it in flight: nothing of it lands
+        # after the new one.
+        new = resp.get_json()["data"]["request_id"]
+        assert sent[-1] == new and sent.count(new) == 1
         assert outcome["status"] == "idle" and outcome["last_event"] == "superseded"
 
     def test_a_service_that_will_not_start_is_an_error(self, api_v3_client, service, clock):
@@ -478,13 +481,20 @@ class TestRealSocket:
         assert data["transport"] == "socket"
         assert [x.request_id for x in live.drain()] == [data["request_id"]]
 
-    def test_a_display_that_went_away_is_an_error(self, api_v3_client, service, live,
+    def test_a_display_that_went_away_is_given_up_on(self, api_v3_client, service, live,
                                                   monkeypatch):
+        from web_interface import on_demand_dispatch
         monkeypatch.setattr(f"{DISPLAY}.ON_DEMAND_SOCKET_WAIT_RUNNING_SECONDS", 0.3)
+        monkeypatch.setattr(on_demand_dispatch, "RETRY_INTERVAL", 0.01)
         live.close()
         resp = api_v3_client.post(START_URL, json={"plugin_id": "weather"})
-        assert resp.status_code == 503
+        # The service still reads as running: answered at once, and the
+        # web process gives up once the wait is over.
+        assert resp.status_code == 202
         assert resp.get_json()["data"]["socket_error"] == "no_socket"
+        d = on_demand_dispatch.current()
+        assert _until(lambda: not d.pending())
+        assert d.status()["error"] == "start-timeout"
         assert _mailbox_writes(service["cache"]) == []
 
     def test_a_full_queue_is_reported_not_mailed(self, api_v3_client, service, monkeypatch):