refactor(query): split datetime helpers by responsibility

[forsaken628]

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Summary

• Move shared Date and Timestamp conversion primitives into their owning expression type modules and remove the cross-layer date_helper utility.
• Split the monolithic datetime implementation into focused arithmetic, conversion, extraction, formatting, rounding, and time-slice modules.
• Update formatting, interval, window, dictionary, and task-history consumers to use the new APIs while preserving existing date-boundary and timezone behavior.

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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