kafka: decouple admin tests from Sarama

[3AceShowHand]

Parent

• Tracking Kafka enhancement and migrate to franz-go #5881

What to build

Make Kafka admin tests depend on a library-independent AdminClient contract. Preserve missing-config handling, Admin API error classification, authorization fallback, and resource cleanup without exposing Sarama types through test interfaces.

Acceptance criteria

• ClusterAdminClient is renamed to AdminClient.
• Authorization failures use a library-independent Kafka admin error while ErrKafkaAdminAPI remains the general Admin API error.
• Admin and topic manager tests do not import or refer to Sarama.
• Missing config, authorization fallback, general Admin API failure, and resource cleanup remain covered.
• Sarama-specific admin interfaces and mocks are confined to adapter-only tests; common AdminClient and topic manager behavior tests use TiCDC-owned mocks.