Announce an acknowledged node failure on the bus

Clearing a node's last error on the engine published nothing, so a second
browser kept the marker until its next snapshot. One event carries the
qualified node; the receiving client drops the marker without refetching.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Gf7WaExcJ9bs3kfJXB3nK

backend/tests/api/routes/test_flows.py

@@ -1,10 +1,12 @@
 from datetime import UTC, datetime
 
+import pytest
 from fastapi.testclient import TestClient
 from sqlalchemy import func
 from sqlmodel import Session, select
 
 from fluksio.core.config import settings
+from fluksio.flow.events import event_bus
 from fluksio.models import Run, RunArtifact, RunMetric, RunNode
 
 PREFIX = f"{settings.API_V1_STR}/flows"
@@ -422,6 +424,34 @@ def test_a_node_that_raises_answers_with_its_error(
     client.delete(f"{PREFIX}/failing", headers=superuser_token_headers)
 
 
+def test_acknowledging_a_failure_is_announced_on_the_bus(
+    client: TestClient,
+    superuser_token_headers: dict[str, str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Another browser holds the same marker; the event is what clears it."""
+    saved = client.put(
+        f"{PREFIX}/acked", headers=superuser_token_headers, json=a_flow("acked")
+    ).json()
+    client.post(
+        f"{PREFIX}/acked/publish",
+        headers=superuser_token_headers,
+        json={"version": saved["definition"]["version"]},
+    )
+
+    published: list[dict] = []
+    monkeypatch.setattr(event_bus, "publish", published.append)
+    response = client.post(
+        f"{PREFIX}/acked/nodes/sensor/acknowledge", headers=superuser_token_headers
+    )
+
+    assert response.status_code == 200
+    event = next(e for e in published if e["type"] == "node_error_acknowledged")
+    assert event["node"] == "acked.sensor"
+
+    client.delete(f"{PREFIX}/acked", headers=superuser_token_headers)
+
+
 def test_renaming_a_flow_leaves_nothing_under_the_old_name(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None: