Fix ClickHouse cluster CTAS execution

[0xhanh]

Description

Fix ClickHouse CTAS behavior in cluster mode.

Cluster CTAS now follows the same two-step flow as ClickHouse Cloud:

1. Create the table with EMPTY and ON CLUSTER.
2. Run a single client-side INSERT ... SELECT.

This prevents ON CLUSTER ... AS SELECT from executing the source query on every cluster node and duplicating data. A regression test covers the generated cluster-mode SQL.

Test Plan

• Ran .venv/bin/python -m pytest tests/core/engine_adapter/test_clickhouse.py -q — 35 passed.
• Ran Ruff format and lint checks for the changed files.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the [DCO](DCO)