fix(seer): Return the resolver response on continue, not a blanket 400

Continue ignored resolve_seer_run_state_id's error response and flattened
every unresolved id to 400 Invalid run_id, inconsistent with GET on the
same id. Return the resolver's response instead, so a not-found run is a
404 and a malformed id stays a 400.

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

Author: trevor-e

src/sentry/seer/endpoints/organization_seer_agent_chat.py

@@ -264,7 +264,7 @@ def post(
             if run_id:
                 seer_run_id_or_response = resolve_seer_run_state_id(run_id, organization)
                 if isinstance(seer_run_id_or_response, Response):
-                    return Response({"detail": "Invalid run_id"}, status=400)
+                    return seer_run_id_or_response
                 seer_run_state_id = seer_run_id_or_response
                 # Continue existing conversation
                 result_run_id = client.continue_run(

tests/sentry/seer/endpoints/test_organization_seer_agent_chat.py

@@ -254,14 +254,27 @@ def test_post_continue_with_uuid_resolves(self, mock_client_class: MagicMock) ->
         assert mock_client.continue_run.call_args.kwargs["run_id"] == 555
 
     @patch("sentry.seer.endpoints.organization_seer_agent_chat.SeerAgentClient")
-    def test_post_continue_with_unknown_uuid_returns_400(
+    def test_post_continue_with_unknown_uuid_returns_404(
         self, mock_client_class: MagicMock
     ) -> None:
         mock_client = MagicMock()
         mock_client_class.return_value = mock_client
 
         response = self.client.post(f"{self.url}{uuid.uuid4()}/", {"query": "More"}, format="json")
 
+        assert response.status_code == 404
+        assert response.data == {"session": None}
+        mock_client.continue_run.assert_not_called()
+
+    @patch("sentry.seer.endpoints.organization_seer_agent_chat.SeerAgentClient")
+    def test_post_continue_with_garbage_run_id_returns_400(
+        self, mock_client_class: MagicMock
+    ) -> None:
+        mock_client = MagicMock()
+        mock_client_class.return_value = mock_client
+
+        response = self.client.post(f"{self.url}not-a-real-id/", {"query": "More"}, format="json")
+
         assert response.status_code == 400
         mock_client.continue_run.assert_not_called()