[13.x] Add UnitEnum support to Application and Translator locale methods

[juan-flores077]

Laravel Version

13.x

PHP Version

8.3.0

Database Driver & Version

No response

Description

The recent enum-support pass across the framework's manager classes
(#59614 base Manager::driver(), #59637 CacheManager, #59645 MailManager,
#59646 AuthManager, #59389 QueueManager, #59391 LogManager, #59420
BroadcastManager) made it natural to use enums as driver/connection/store
identifiers everywhere — except for the application's locale.

Notification::locale() already accepts a UnitEnum (#55922), but the
underlying primitives still require a string:

• Illuminate\Foundation\Application::setLocale($locale)
• Illuminate\Foundation\Application::setFallbackLocale($fallbackLocale)
• Illuminate\Foundation\Application::isLocale($locale)
• Illuminate\Translation\Translator::setLocale($locale)
• Illuminate\Translation\Translator::setFallback($fallback)

This forces users with a typed Locale enum to call ->value everywhere:

enum Locale: string {
    case English  = 'en';
    case Japanese = 'ja';
    case Korean   = 'ko';
}

App::setLocale(Locale::Japanese->value);     // current — clunky
App::setLocale(Locale::Japanese);            // proposed — consistent with Notification::locale()

### Steps To Reproduce

`Application::setLocale`, `Application::setFallbackLocale`,
`Application::isLocale`, `Translator::setLocale`, `Translator::setFallback`
silently accept enum objects (or TypeError, depending on PHP version) and
store them as objects in config, breaking string comparisons throughout
the request lifecycle.