feat: add exploration tools (warehouse_explore, tables_inspect, table_sample)

[kulvirgit]

Summary

• Adds warehouse_explore tool to list all schemas, tables, and column names in one call
• Adds tables_inspect tool to get column names and types for specific tables
• Adds table_sample tool to get sample rows from tables to see actual data values
• Updates builder prompt to mandate data exploration before writing SQL

Test plan

• Verify warehouse_explore returns schemas/tables/columns from connected warehouse
• Verify tables_inspect returns column types for specified tables
• Verify table_sample returns sample rows
• Confirm builder agent uses exploration tools before writing SQL

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