fix: send 404 when a sessions is not found (#1234)

Closes #1209.

Author: leafty

components/renku_data_services/notebooks/blueprints.py

@@ -115,7 +115,9 @@ def get_one(self) -> BlueprintFactoryResponse:
         async def _handler(_: Request, user: AuthenticatedAPIUser | AnonymousAPIUser, session_id: str) -> HTTPResponse:
             session = await self.nb_config.k8s_v2_client.get_session(session_id, user.id)
             if session is None:
-                raise errors.ValidationError(message=f"The session with ID {session_id} does not exist.", quiet=True)
+                raise errors.MissingResourceError(
+                    message=f"The session with ID {session_id} does not exist.", quiet=True
+                )
             return json(session.as_apispec().model_dump(exclude_none=True, mode="json"))
 
         return "/sessions/<session_id>", ["GET"], _handler

test/bases/renku_data_services/data_api/test_notebooks.py

@@ -303,3 +303,18 @@ async def test_start_server(
     _, res = await sanic_client.delete(f"/api/data/sessions/{server_name}", headers=authenticated_user_headers)
 
     assert res.status_code == 204, res.text
+
+
+@pytest.mark.xdist_group("sessions")  # Needs to run on the same worker as the rest of the sessions tests
+@pytest.mark.sessions
+@pytest.mark.asyncio
+async def test_not_found_server(
+    sanic_client: SanicASGITestClient,
+    authenticated_user_headers: dict[str, str],
+    notebooks_fixtures,
+):
+    server_name = "unknown_server"
+
+    _, res = await sanic_client.get(f"/api/data/sessions/{server_name}", headers=authenticated_user_headers)
+
+    assert res.status_code == 404, res.text