Pass flexible_cluster_mode config to Repository in CacheManager

Repository::flexible() references the flexible_cluster_mode config,
but CacheManager::repository() was using Arr::only($config, ['store']),
which prevented flexible_cluster_mode from being passed to Repository.

Changes:
- Modified Arr::only($config, ['store', 'flexible_cluster_mode'])
- This ensures flexible_cluster_mode config is properly passed to Repository

Test results:
- Local: 75 tests, 302 assertions all passed
- Valkey Serverless: 3/3 patterns all succeeded
  - phpredis + default: SUCCESS
  - predis + default: SUCCESS
  - predis + clusters: SUCCESS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: hayase_yasuhiro

src/Illuminate/Cache/CacheManager.php

@@ -369,7 +369,7 @@ protected function getSession()
      */
     public function repository(Store $store, array $config = [])
     {
-        return tap(new Repository($store, Arr::only($config, ['store'])), function ($repository) use ($config) {
+        return tap(new Repository($store, Arr::only($config, ['store', 'flexible_cluster_mode'])), function ($repository) use ($config) {
             if ($config['events'] ?? true) {
                 $this->setEventDispatcher($repository);
             }