[12.x] Add missing conditional validation rule builders

[SanderMuller]

This PR adds the missing unless counterparts to the existing conditional validation rule builders (Rule::requiredIf(), Rule::excludeIf(), Rule::prohibitedIf()).

Motivation

Laravel provides Rule::requiredIf(), Rule::excludeIf(), and Rule::prohibitedIf() for closure/boolean-based conditional validation. However, the unless variants are missing. While you can negate the condition (Rule::requiredIf(! $isFree)), the unless form reads more naturally in many contexts and matches the naming convention users expect when they see the If variants exist.

Example usage

// Before: negate the condition manually
'coupon' => [Rule::excludeIf(! $hasSubscription), 'string']
'terms' => [Rule::requiredIf(! $isAdmin), 'accepted']
'legacy_field' => [Rule::prohibitedIf(! $allowLegacy)]

// After: express intent directly
'coupon' => [Rule::excludeUnless($hasSubscription), 'string']
'terms' => [Rule::requiredUnless($isAdmin), 'accepted']
'legacy_field' => [Rule::prohibitedUnless($allowLegacy)]

Works with closures too:

Rule::requiredUnless(fn () => $this->user()->isAdmin())
Rule::excludeUnless(fn () => $this->hasActiveSubscription())

Added

• Rule::excludeUnless($callback) - excludes the field when condition is false
• Rule::requiredUnless($callback) - requires the field when condition is false
• Rule::prohibitedUnless($callback) - prohibits the field when condition is false