fix: load paginated run history messages (#3305)

backend/tests/_run_message_pagination_helpers.py

@@ -0,0 +1,16 @@
+from fastapi.testclient import TestClient
+
+
+def assert_run_message_page(
+    client: TestClient,
+    url: str,
+    *,
+    expected_seq: list[int],
+    has_more: bool = True,
+) -> None:
+    response = client.get(url)
+
+    assert response.status_code == 200
+    body = response.json()
+    assert body["has_more"] is has_more
+    assert [m["seq"] for m in body["data"]] == expected_seq