GET /runs/{id}/nodes/{node}/outputs serves what a node returned

The stage cache's stored outputs, as kept (up to 256 kB of JSON). A node
that kept none answers 404 saying why. Listed in the API docs.

backend/tests/api/routes/test_runs.py

@@ -1115,6 +1115,37 @@ def test_a_curve_whose_run_is_gone_is_empty_rather_than_an_error(
     assert node["curve_lost"] is True
 
 
+def test_a_node_answers_with_what_it_returned(client, superuser_token_headers):
+    """What the stage cache stored, and a 404 that says why for a node without."""
+    run_id = new_run_id()
+    with Session(db_engine) as session:
+        session.add(
+            Run(id=run_id, flow="study", status="ok", created_at=datetime.now(UTC))
+        )
+        session.add(RunNode(run_id=run_id, node="fit", outputs='{"study.loss":0.5}'))
+        session.add(RunNode(run_id=run_id, node="plot", status="error"))
+        session.commit()
+    url = f"{settings.API_V1_STR}/runs/{run_id}/nodes"
+    try:
+        kept = client.get(f"{url}/fit/outputs", headers=superuser_token_headers)
+        assert kept.json() == {"study.loss": 0.5}
+
+        none = client.get(f"{url}/plot/outputs", headers=superuser_token_headers)
+        assert none.status_code == 404
+        assert "kept no outputs" in none.json()["detail"]
+
+        absent = client.get(f"{url}/nope/outputs", headers=superuser_token_headers)
+        assert absent.status_code == 404
+    finally:
+        with Session(db_engine) as session:
+            for row in session.exec(
+                select(RunNode).where(col(RunNode.run_id) == run_id)
+            ).all():
+                session.delete(row)
+            session.delete(session.get(Run, run_id))
+            session.commit()
+
+
 def _metric(run_id: str, name: str, step: int, value: float, ts: float) -> RunMetric:
     return RunMetric(run_id=run_id, name=name, step=step, value=value, ts=ts)