feat(query): support cascading hierarchical grouping sets

[forsaken628]

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Summary

• add the opt-in enable_cascading_grouping_sets setting to derive each ROLLUP level from its closest strict superset, while keeping the existing shared-base strategy as the default
• preserve distributed execution for scalar Materialized CTE producers by inserting a constant-key GlobalHash exchange after physical optimization
• move Materialized CTE distribution normalization into a dedicated post-Cascades optimizer, keeping it independent of the grouping-sets rule and Cascades search
• preserve GROUP BY () semantics for empty input instead of converting the scalar aggregate into a keyed aggregate

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

AI assistance

• AI usage: An AI coding agent assisted with implementation.
• Responsible human: @forsaken628
• The responsible human has read every line of this diff and can explain each change

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