mcp: hint agents to prefer indexed objects

[bobbyiliev]

As discussed adding a short instructions block to the MCP agent endpoint's initialize response, nudging agents to prefer indexed objects over materialized views. Pairs with the indexedColumns hint already surfaced by get_data_product_details so the agent has both session-level guidance and per-tool signals for picking the fast path.

[jubrad]

@bobbyiliev I'm wondering if we need to be very explicit about either

1. only showing indexes on the cluster the agent is connected to
2. stressing that indexes are only accessible when they are on the current cluster, and that if a view with an index is queried on a cluster that does not have the index the index will not be used.