Retire a shared node in use with ?detach=true

DELETE /flows/library/{name} still answers 409 while nodes use the source.
With detach=true each of them keeps a copy of the code, in the published
flow and the draft, the source is deleted and the running users rebuild.

backend/tests/api/routes/test_flows.py

@@ -550,3 +550,34 @@ async def test_the_live_socket_ends_when_its_client_goes(
     await asyncio.sleep(0.05)
     parked = [t for t in asyncio.all_tasks() if "Queue.get" in repr(t.get_coro())]
     assert parked == []
+
+
+def test_a_shared_node_in_use_is_retired_only_with_detach(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    client.put(
+        f"{PREFIX}/shared", headers=superuser_token_headers, json=a_flow("shared")
+    )
+    client.put(
+        f"{PREFIX}/shared/nodes/sensor/source",
+        headers=superuser_token_headers,
+        json={"code": WORKING_NODE},
+    )
+    client.post(
+        f"{PREFIX}/shared/nodes/sensor/share",
+        headers=superuser_token_headers,
+        json={"lib_name": "reading"},
+    )
+
+    url = f"{PREFIX}/library/reading"
+    assert client.delete(url, headers=superuser_token_headers).status_code == 409
+    response = client.delete(f"{url}?detach=true", headers=superuser_token_headers)
+    assert response.status_code == 200
+
+    assert client.get(f"{PREFIX}/library", headers=superuser_token_headers).json() == []
+    source = client.get(
+        f"{PREFIX}/shared/nodes/sensor/source", headers=superuser_token_headers
+    )
+    assert source.json()["code"] == WORKING_NODE
+
+    client.delete(f"{PREFIX}/shared", headers=superuser_token_headers)