fix: add SQLite connector cloning for multi-source support

[Yax]

Fixes #115 where multiple SQLite databases in multi-source configuration would overwrite each other, leaving only the last database accessible.

Root cause: ConnectorRegistry.getConnectorForDSN() returned singleton instances, so each connect() call overwrote the previous connection.

Solution:

• Add optional clone() method to Connector interface
• Implement clone() in SQLiteConnector to create separate instances
• Update ConnectorManager to use clone() for multi-source configs
• Other connectors remain unchanged (not tested with multi-source)

Tests:

• Add comprehensive integration tests for multiple SQLite sources
• All 9 new tests pass, verifying independent database operations
• Existing 31 SQLite integration tests still pass
• Build succeeds with no TypeScript errors

[tianzhou]

LGTM. Thanks for the fix