[EM] Compact get-nodes responses in MCP

[astandrik]

Users report that ydb-get-nodes can return an excessively large response that is difficult for both the assistant and the user to work with.

Make the default response compact and bounded, similar to the describe work in ydb-em#140 and ydb-em#235. Keep the fields needed for common node diagnostics and allow additional detail to be requested explicitly. Apply this in the EM MCP tool so all clients benefit.

Tracked in the UI backlog; implementation belongs to the EM MCP backend. The ydb-em GitHub repository is archived.

Done when

• Reproduce the problem with a representative large response and record its size before and after the change.
• Default responses stay manageable for large clusters without losing essential diagnostic information.
• Explicit field selection and pagination remain usable for retrieving more detail.
• Any omitted nodes or details are clearly indicated; the caller can tell whether the result is complete and how to request the rest.
• Tests cover large responses, explicit detailed requests, pagination, and empty results.