Feature: new ignore operation: LowCardinality(String) vs String

[hh2110]

Summary

• Add support for ignoring LowCardinality differences in column comparisons
• Enhance column equivalence logic to handle IgnoreStringLowCardinalityDifferences flag
• Add comprehensive test coverage for LowCardinality normalization functionality

Co-Authored-By: Claude noreply@anthropic.com

---

Note

Adds an ignorable operation to treat LowCardinality(String) and String as equivalent during diffs, wiring it through comparisons/normalization and adding comprehensive tests.

• Core Diffing/Infra Map:
  • Extend compute_table_columns_diff and columns_are_equivalent to accept ignore_ops, honoring IgnoreStringLowCardinalityDifferences.
  • Use ClickHouse-aware type comparison with low-cardinality normalization.
  • Update simple diff helpers and call sites to pass &[] default.
• ClickHouse Diff Strategy:
  • Add ignore_low_cardinality to column_types_are_equivalent, json_options_are_equivalent, and nested_are_equivalent.
  • Introduce normalize_column_for_low_cardinality_ignore to strip LowCardinality annotations when ignoring.
• ClickHouse Mod (execution/normalization):
  • Add IgnorableOperation::IgnoreStringLowCardinalityDifferences and handle it in normalize_table_for_diff to strip LowCardinality annotations.
• CLI:
  • Update external table change detection to use new compute_table_columns_diff(.., &[]).
• Tests:
  • Add extensive unit/integration tests covering low-cardinality ignore behavior across columns, nested/JSON types, normalization, and table-level diffs.

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