feat(query): propagate column statistics through UNION ALL

[forsaken628]

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Summary

• Derive UNION ALL output column statistics by mapping child columns to output symbols, applying binder-inserted coercions, and merging min/max, NULL counts, and NDV.
• Preserve statistics across lossless injective casts while leaving lossy and TRY_CAST conversions unknown.
• Estimate merged NDV using histogram overlap when available and conservative join-aligned fallbacks otherwise. Clamp NDV to finite output domains to prevent internally inconsistent statistics.

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):

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[chatgpt-codex-connector]

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