Use execute_values instead of execute_batch for better bulk insert performance with PostgresHook

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Summary

This PR optimizes PostgresHook.insert_rows() when fast_executemany=True by switching from execute_batch to execute_values for psycopg2, which provides significantly better bulk insert performance.

Changes

• psycopg2 with fast_executemany=True: Now uses psycopg2.extras.execute_values() instead of execute_batch(). This batches all rows into a single INSERT statement with multiple value tuples, reducing round-trips and improving throughput.

• psycopg3: Falls back to the default DbApiHook.insert_rows() implementation. psycopg3's native executemany already uses pipelining internally, so there's no benefit to a custom implementation—and execute_values is not compatible with psycopg3.

• Format string handling: Both code paths now explicitly set _insert_statement_format to ensure correct SQL generation and self-healing if a previous call failed mid-execution.

Why execute_values over execute_batch?

Method	Behavior
execute_batch	Sends multiple INSERT statements in batches
execute_values	Sends a single INSERT ... VALUES (...), (...), (...) statement

execute_values is typically 2-3x faster for bulk inserts because it minimizes statement parsing overhead and network round-trips.

Testing

• Updated existing tests to verify execute_values is called instead of execute_batch
• Added new test to verify psycopg3 correctly falls back to the default implementation even when fast_executemany=True

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Claude Opus 4.6

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