Feature/date format rule

[litedatum]

1. Research and implement SQLite date parsing strategies:
   • Custom date validation using SUBSTR and CAST functions
   • Format pattern validation using SQLite's limited regex support
2. Implement PostgreSQL date parsing using TO_DATE:
   • Map common format patterns to PostgreSQL format strings
   • Handle timezone and locale considerations
3. Extend ValidityExecutor._generate_date_format_sql() for multi-database support
4. Update DatabaseDialect.is_supported_date_format() logic
5. Create comprehensive test suite for date format validation:
   • Test suite with multiple format patterns across all databases
   • Edge case testing (leap years, invalid dates, boundary conditions)
6. Performance optimization for date validation queries
7. Update documentation with supported date format patterns per database
8. Regression test and documentation

Resolve #40