Add createSchemaTools(graph) factory that returns 7 LangChain-compatible
tools for LLM agents to explore database schemas: schema_search,
schema_summary, schema_table_info, schema_find_join_path,
schema_neighbors, schema_relationships, and schema_referencing_tables.

Also includes prior API improvements:
- Rename GroupSummary.name to groupName
- Add table descriptions (notes) to getSummary output
- Wrap searchSchema return in SearchResultResponse with tableDescriptions
- Export SummaryTable and SearchResultResponse types