feat(config): add per-client class override option

[guillaumedelre]

Summary

• Add an optional class key to the client configuration to allow specifying a custom client class per client
• The option takes precedence over the global snc_redis.class.client / snc_redis.class.phpredis_client settings
• Works for predis, phpredis and relay client types

Motivation

Closes #729 — there was no way to override the instantiated class for a specific client without writing a compiler pass. This adds a straightforward config option:

snc_redis:
    clients:
        default:
            type: predis
            dsn: redis://localhost
            class: App\Redis\CustomPredisClient   # must extend Predis\Client
        cache:
            type: phpredis
            dsn: redis://localhost
            class: App\Redis\CustomRedis          # must extend Redis

Test plan

• testPredisCustomClass — verifies the DI definition uses the configured class for a predis client
• testPhpRedisCustomClass — verifies the DI definition uses the configured class for a phpredis client
• Full test suite: no regressions (43 tests in SncRedisExtensionTest, same 5 pre-existing failures due to ext-redis unavailable locally)
• PHPCS clean
• Psalm: no new errors vs baseline