[13.x] Add enum support to ChannelManager

[sumaiazaman]

Summary

This PR adds enum support to ChannelManager::channel() and ChannelManager::deliverVia() using the enum_value() helper, aligning it with other manager classes that already support this pattern.

Context

Several manager classes already accept enums for connection/driver names:

Class	Supports Enums
Manager::driver()	✅ (merged in #59659)
CacheManager::store()	✅ (merged in #59637)
AuthManager::guard()	✅ (merged in #59646)
MailManager::mailer()	✅ (merged in #59645)
QueueManager::connection()	✅ (merged in #59389)
LogManager::channel()	✅ (merged in #59391)
FilesystemManager::disk()	✅
RedisManager::connection()	✅
ChannelManager::channel()	❌
ChannelManager::deliverVia()	❌

This inconsistency means developers who define notification channel names as enums cannot pass them directly to Notification::channel() like they can with Queue::connection() or Cache::store(). More importantly, passing an enum to deliverVia() silently stores the enum object in $defaultChannel instead of its string value, causing a mismatch when the value is later used as a driver key.

Solution

• Updated @param docblock on channel() to accept \UnitEnum|string|null — the parent Manager::driver() already calls enum_value(), so no logic change is needed here
• Added enum_value() call in deliverVia() to ensure the string value is stored in $defaultChannel
• Updated @param docblock on deliverVia() to accept \UnitEnum|string

Example

enum NotificationChannel: string
{
    case Mail     = 'mail';
    case Database = 'database';
}

// Before: deliverVia() silently stored the enum object, breaking driver resolution
Notification::deliverVia(NotificationChannel::Database);

// After: works as expected
Notification::channel(NotificationChannel::Mail)->send(...);
Notification::deliverVia(NotificationChannel::Database);

Why This Doesn't Break Existing Features

• enum_value() returns the input unchanged for strings and null — all existing calls behave identically
• The channel() change is docblock-only; the underlying Manager::driver() already handles enum resolution
• Follows the exact same pattern already established across CacheManager, AuthManager, MailManager, QueueManager, LogManager, FilesystemManager, RedisManager, and the base Manager class

Changes

• src/Illuminate/Notifications/ChannelManager.php — Added enum_value() in deliverVia(); updated docblocks on channel() and deliverVia()
• tests/Notifications/NotificationChannelManagerTest.php — Added 2 test cases

Test Plan

• testChannelManagerCanResolveBackedEnumChannel — Verifies backed enum resolves to the correct channel driver
• testDeliverViaAcceptsBackedEnum — Verifies the string value is stored in $defaultChannel, not the enum object
• Existing tests pass (no breaking changes)