test(db): add comprehensive unit tests for schema module

- Test ColumnInfo, ForeignKeyInfo, TableInfo dataclasses
- Test SchemaInfo with table lookup and serialization
- Test SchemaIntrospector caching and prompt serialization
- Test get_sample_data success and error handling
- Test various serialization options (FK, types, max_tables)

Coverage: 23 tests for db/schema.py

Author: Sakeeb91