[13.x] Add Redis cache prefix clearing

[DGarbs51]

Summary

This PR adds an opt-in way for Redis cache stores to clear only keys managed by the configured Laravel cache prefix instead of flushing the selected Redis database.

It introduces:

• Illuminate\Contracts\Cache\CanFlushPrefix
• RedisStore::flushPrefix()
• Repository::flushPrefix() / supportsFlushingPrefix()
• php artisan cache:clear --prefix
• cache.stores.redis.flush_scope, configurable with REDIS_CACHE_FLUSH_SCOPE=prefix

The existing Redis cache flush() behavior is unchanged and continues to call FLUSHDB unless the new prefix mode is explicitly requested.

Why

Laravel's default Redis configuration has historically encouraged isolating cache data in a separate logical Redis database. That is increasingly difficult with managed Redis-compatible services:

• Redis documents that Redis Cluster only supports database zero and that SELECT cannot be used in cluster mode: https://redis.io/docs/latest/commands/select/
• Redis also documents that Redis Software and Redis Cloud do not support shared logical databases; SELECT is accepted only for compatibility and does not perform an operation: https://redis.io/docs/latest/commands/select/
• AWS documents that ElastiCache Serverless abstracts the cluster topology and that FLUSHDB / FLUSHALL always flush all keys across the cluster: https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/SupportedCommands.html

That means applications running on modern Redis Cluster, Redis Cloud, Redis Software, or ElastiCache Serverless deployments need a first-party way to clear only Laravel cache keys without clearing unrelated data that shares database zero.

Backwards Compatibility

This is intended to be backwards compatible:

• RedisStore::flush() still calls flushdb().
• cache:clear still uses the existing flush behavior by default.
• Prefix clearing is only used when cache:clear --prefix is passed or when cache.stores.redis.flush_scope is set to prefix.
• The new cache store capability is an optional contract, so existing cache store implementations are not required to add new methods.
• Prefix clearing refuses to run when the Redis cache store has an empty prefix.

Tests

• vendor/bin/pint --dirty
• git diff --check
• vendor/bin/phpunit tests/Cache/CacheRedisStoreTest.php tests/Cache/ClearCommandTest.php tests/Cache/CacheEventsTest.php
• vendor/bin/phpunit tests/Cache

The full cache suite passes. PHPUnit reports the existing notices/skips from the suite: 8 notices and 13 skipped tests.