feat: add DateTime sentinel value support to test_sentinel_value_approach

[vincent-herlemont]

Summary

• Added DateTime wrapper with ToKey implementation for sentinel value pattern
• Replaced NaiveDate with DateTime for better precision and chrono compatibility
• Added helper function and constant for sentinel date creation
• Updated test to demonstrate queryable date/time "null" values

Changes

• DateTimeWrapper struct: Implements ToKey using timestamp_millis() method
• SENTINEL_DATE_TIMESTAMP constant: Defined timestamp for year 1000 as sentinel
• create_sentinel_date() helper: Encapsulates sentinel date creation
• Updated test data: Uses proper DateTime values with timestamps
• Enhanced test coverage: Demonstrates date range queries with sentinels

Benefits

• Follows the official chrono DateTime example pattern from codebase docs
• Enables efficient querying of date/time fields with "null" representation
• Avoids Option limitations where None values can't be indexed/queried
• Provides better precision with millisecond timestamps vs day-based ordering

Test Results

All tests pass, including the enhanced test_sentinel_value_approach that now covers:

• Querying for items with sentinel dates
• Date range queries excluding sentinel values
• Helper functions for checking non-sentinel dates