fix: case-insensitive validation for custom field options

[ManukMinasyan]

Replace Filament's distinct() with custom closure rule using Collection duplicates() for case-insensitive comparison. Prevents database unique constraint violations when adding options like "AB" and "ab".

[Copilot]

Pull request overview

This PR enhances validation for custom field options by implementing case-insensitive duplicate detection. The change prevents database unique constraint violations that would occur when users attempt to create options that differ only in case (e.g., "Active" and "active").

Changes:

• Replaced Filament's built-in distinct() validation with a custom closure that performs case-insensitive comparison using Laravel Collection's duplicates() method
• Added test coverage to verify case-insensitive validation works correctly

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Filament/Management/Schemas/FieldForm.php	Replaced distinct() with custom validation closure that uses mb_strtolower() and Collection duplicates() for case-insensitive duplicate detection
tests/Feature/Admin/Pages/CustomFieldsValidationTest.php	Added test case to verify option names are validated as unique in a case-insensitive manner

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