2.0.5

Add Laravel validation rules for role assignment validation

Features:
- NotAssignedTo rule: validates entity is not already assigned
- AssignedTo rule: validates entity has existing role assignment
- Comprehensive test suite with 18 tests covering edge cases
- Updated Laravel integration docs with validation examples
- Code formatted with Laravel Pint

Benefits:
- Prevents duplicate role assignments
- Validates existing assignments before operations
- Seamless Laravel validation system integration
- Type-safe using Porter entity contracts