Clean up summary API usage

While investigating a user's schema sync issue I noticed some code that could use some cleaning up.

* Calls with `mode=detailed` but doesn't use extra fields
* Does not validate response structure
* Performs unchecked property access on response

https://github.com/aws/graph-explorer/blob/b09962331c82802ea35593d6f381cc26f6e2cb01/packages/graph-explorer/src/connector/gremlin/gremlinExplorer.ts#L47-L69

Open packages/graph-explorer/src/connector/gremlin/gremlinExplorer.ts at lines 47-69 and trace the summary API call and response handling. Review how the response is used, then make the cleanup address the detailed-mode request, response validation, and unchecked property access; done means these three concerns are resolved without changing the intended summary behavior.

Written by the indexing model from the issue text.