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

[bipinks]

This PR extends the recent enum-support pass across the framework's manager classes (#59614, #59637, #59645, #59646, #59389, #59391, #59420) to the application's locale primitives.

Notification::locale() already accepts a UnitEnum (#55922), but the underlying primitives still required a string, forcing users with a typed Locale enum to call ->value everywhere.

Changes

The following methods now accept \UnitEnum|string and normalize via the existing Illuminate\Support\enum_value() helper:

• Illuminate\Foundation\Application::setLocale()
• Illuminate\Foundation\Application::setFallbackLocale()
• Illuminate\Foundation\Application::isLocale()
• Illuminate\Translation\Translator::setLocale()
• Illuminate\Translation\Translator::setFallback()

Contracts (Illuminate\Contracts\Foundation\Application, Illuminate\Contracts\Translation\Translator) updated with matching docblocks.

Usage

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

App::setLocale(Locale::Japanese);        // now works
App::setFallbackLocale(Locale::English); // now works
App::isLocale(Locale::Japanese);         // now works

Both BackedEnum (uses ->value) and pure UnitEnum (uses ->name) are supported, matching the behavior of enum_value() used elsewhere in the framework.

Tests

Added unit tests covering backed and pure enum inputs for all five methods:

• FoundationApplicationTest::testSetLocaleAcceptsBackedEnum
• FoundationApplicationTest::testSetLocaleAcceptsUnitEnum
• FoundationApplicationTest::testSetFallbackLocaleAcceptsBackedEnum
• FoundationApplicationTest::testIsLocaleAcceptsEnum
• TranslationTranslatorTest::testSetLocaleAcceptsBackedEnum
• TranslationTranslatorTest::testSetLocaleAcceptsUnitEnum
• TranslationTranslatorTest::testSetFallbackAcceptsBackedEnum
• TranslationTranslatorTest::testSetFallbackAcceptsUnitEnum

Closes #59667