Support multiple Kafka broker addresses in endpoint URL

[badaverse]

Please ensure you adhere to every item in this list

• This PR was pre-approved by the project maintainer
• I have self-reviewed the code
• I have added all necessary tests

Describe your changes

Summary

• Add multiple broker support for Kafka endpoints using comma-separated addresses (kafka://broker1:9092,broker2:9092/topic)
• Update SETHOOK URL split logic to correctly distinguish Kafka broker commas from endpoint fallback commas
• Replace single Host/Port fields with Brokers []string in the Kafka endpoint struct

Changes

• internal/endpoint/endpoint.go: Replace Host string + Port int with Brokers []string in the Kafka struct. Update the URL parser to split the host portion on commas and collect each host:port pair into the slice.
• internal/endpoint/kafka.go: Pass conn.ep.Kafka.Brokers directly to sarama.NewSyncProducer instead of constructing a single URI string.
• internal/server/hooks.go: Introduce splitEndpointURLs() to replace naive comma splitting. Parts without a scheme prefix (:// or Endpoint=) are joined back to the preceding URL, so kafka://b1:9092,b2:9092/topic,http://backup/hook correctly splits into two endpoints.

Backward compatibility

• Single broker URLs (kafka://host:9092/topic) continue to work as before.
• Existing SETHOOK fallback behavior with multiple endpoints is preserved.

Issue number and link

#805