This commit adds detailed debug logging throughout the message flow to help
diagnose the 'Message content mismatch' error observed in GitHub Actions:
1. Mock backend flow (unit tests):
- [MOCK_STORE]: Log when storing messages to mock handler
- [MOCK_RETRIEVE]: Log when retrieving messages from mock handler
2. Real SMQ backend flow (GitHub Actions):
- [LOG_BUFFER_UNMARSHAL]: Log when unmarshaling LogEntry from log buffer
- [BROKER_SEND]: Log when broker sends data to subscriber clients
3. Gateway decode flow (both backends):
- [DECODE_START]: Log message bytes before decoding
- [DECODE_NO_SCHEMA]: Log when returning raw bytes (schema disabled)
- [DECODE_INVALID_RV]: Log when RecordValue validation fails
- [DECODE_VALID_RV]: Log when valid RecordValue detected
All new logs use glog.Infof() so they appear without requiring -v flags.
This will help identify where data corruption occurs in the CI environment.