fix(clickhouse): strip virtual catalogs from executed queries and inserts

[mday-io]

Description

When a ClickHouse gateway is used alongside catalog-aware gateways, SQLMesh injects a virtual catalog (__<gateway>__ by default, or the configured virtual_catalog) so every model has a 3-part name. ClickHouse doesn't support catalogs, so that prefix has to be stripped before SQL reaches the engine. Some DDL paths already handled this (create_view, alter_table, delete_from's target table, create_schema), but the queries themselves still carried the catalog. As a result, INSERT ... SELECT, CREATE TABLE ... AS SELECT and subqueries inside DELETE ... WHERE all failed.

This PR strips the virtual catalog centrally in ClickhouseEngineAdapter._to_sql, so it covers every rendered statement:

• Any table or column reference whose catalog equals the injected _default_catalog has its catalog removed.
• References to other catalogs and string literals are left untouched.
• The caller's expression is never mutated. It is only copied when a reference actually needs rewriting, so large INSERT ... VALUES statements aren't deep-copied for no reason.
• Stripping only happens after inject_virtual_catalog() has run. This matches the existing strip sites, so a ClickHouse-only project with virtual_catalog configured behaves exactly as before.

Test Plan

Added to tests/core/engine_adapter/test_clickhouse.py:

• test_virtual_catalog_stripped_from_execute_queries (parametrized) covers INSERT ... SELECT, a plain SELECT, and a JOIN that mixes the virtual catalog with another catalog plus a string literal containing the catalog name. It also checks that the input expression isn't mutated.
• test_virtual_catalog_stripped_from_ctas_and_delete checks the exact SQL emitted by ctas() and by delete_from() with a subquery.

Both tests fail without the fix. All 38 tests in test_clickhouse.py pass, and so does ruff/mypy on 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