[5.x] Support custom validation rules for asset containers

[duncanmcclean]

Currently, if you add a custom validation rule to an asset container, you'll run into an error:

[2025-12-30 10:56:52] local.ERROR: Method Illuminate\Validation\Validator::validateNewApp\Rules\NotMimes('gif') does not exist. {"userId":"77191f11-a000-41ed-bc9c-61334dbc188d","exception":"[object] (BadMethodCallException(code: 0): Method Illuminate\\Validation\\Validator::validateNewApp\\Rules\\NotMimes('gif') does not exist. at /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Validation/Validator.php:1692)
[stacktrace]
#0 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Validation/Validator.php(686): Illuminate\\Validation\\Validator->__call('validateNewApp\\\\...', Array)
#1 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Validation/Validator.php(481): Illuminate\\Validation\\Validator->validateAttribute('file', 'NewApp\\\\Rules\\\\No...')
#2 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Validation/Validator.php(516): Illuminate\\Validation\\Validator->passes()
#3 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Validation/Validator.php(558): Illuminate\\Validation\\Validator->fails()
#4 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Foundation/Providers/FoundationServiceProvider.php(156): Illuminate\\Validation\\Validator->validate()
#5 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Macroable/Traits/Macroable.php(126): Illuminate\\Http\\Request->Illuminate\\Foundation\\Providers\\{closure}(Array)
#6 /Users/aaron/Code/redacted/vendor/statamic/cms/src/Http/Controllers/CP/Assets/AssetsController.php(84): Illuminate\\Http\\Request->__call('validate', Array)
#7 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Routing/Controller.php(54): Statamic\\Http\\Controllers\\CP\\Assets\\AssetsController->store(Object(Illuminate\\Http\\Request))
#8 /Users/aaron/Code/redacted/vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php(43): Illuminate\\Routing\\Controller->callAction('store', Array)

The error is happening because we're not transforming the new App\Rules\CustomRule() string from the container's YAML file, like we do with blueprint fields.

This PR fixes it by making the Validator::parse() method public and mapping through the validation rules and parsing them when uploading an asset.

Fixes #13407