Support MM/DD/YYYY date format in timestamp column searches

[jguo144]

Description

This PR adds support for MM/DD/YYYY date format in timestamp column searches, while maintaining backward compatibility with YYYY-MM-DD format.

Changes

• Date format validation: Now accepts both YYYY-MM-DD and MM/DD/YYYY formats
• Single date searches: Users can search timestamp columns using either 01/15/2024 or 2024-01-15
• Date range searches: Supports ranges in both formats (e.g., 01/01/2024 to 12/31/2024)
• Dynamic placeholders: Placeholder text adapts based on ckan.datatables.date_format config
  • If set to MM/DD/YYYY: shows "mm/dd/yyyy"
  • Otherwise: shows "yyyy-mm-dd"

Technical Details

• Added _normalize_date() function to convert MM/DD/YYYY to YYYY-MM-DD for SQL queries
• Enhanced _is_valid_date() to validate both formats
• Single date searches are converted to same-day date ranges for accurate timestamp filtering

All changes are backward compatible. Existing YYYY-MM-DD searches continue to work as before.

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca