Reuse DuckDB connection for metadata queries instead of creating new connection per call

[shefeek-jinnah]

Summary

• Refactored DuckdbMetadataProvider to use a single shared DuckDB connection protected by Arc<Mutex<Connection>> instead of creating a new connection for each metadata query
• Reduces connection creation overhead for read-only metadata operations

#44

Changes

• Changed struct field from catalog_path: String to conn: Arc<Mutex<Connection>>
• Connection is now created once in new() and reused across all metadata calls
• Added connection() method that returns MutexGuard<Connection> for thread-safe access
• Renamed open_connection_with_path() to create_connection() for clarity

Rationale

Previously, each of the 14+ metadata methods (get_current_snapshot, list_schemas, get_table_by_name, etc.) opened a new DuckDB connection. Since all operations are read-only and metadata queries are fast, a mutex-protected shared connection is sufficient and eliminates unnecessary connection overhead.