debug: add more logging to message broker for broadcast diagnostics

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/message-router/adapters/output/memory/message_broker.go

@@ -66,13 +66,31 @@ func (a *MessageBrokerAdapter) PublishToSession(
 	defer a.mu.RUnlock()
 
 	prefix := sessionID + ":"
+
+	// Debug: log all session channels
+	var allKeys []string
+	for key := range a.sessionChannels {
+		allKeys = append(allKeys, key)
+	}
+	logger.Debug("PublishToSession looking for channels",
+		zap.String("session_id", sessionID),
+		zap.String("exclude_party", excludeParty),
+		zap.String("prefix", prefix),
+		zap.Int("total_session_channels", len(a.sessionChannels)),
+		zap.Strings("all_keys", allKeys))
+
+	foundCount := 0
 	for key, ch := range a.sessionChannels {
 		if len(key) > len(prefix) && key[:len(prefix)] == prefix {
 			partyID := key[len(prefix):]
 			if partyID == excludeParty {
+				logger.Debug("skipping sender party",
+					zap.String("session_id", sessionID),
+					zap.String("party_id", partyID))
 				continue
 			}
 
+			foundCount++
 			select {
 			case ch <- message:
 				logger.Debug("broadcast message to party",
@@ -88,6 +106,12 @@ func (a *MessageBrokerAdapter) PublishToSession(
 		}
 	}
 
+	if foundCount == 0 {
+		logger.Warn("No parties found for broadcast",
+			zap.String("session_id", sessionID),
+			zap.String("exclude_party", excludeParty))
+	}
+
 	return nil
 }
 
@@ -141,9 +165,17 @@ func (a *MessageBrokerAdapter) SubscribeToSessionMessages(
 
 	key := sessionID + ":" + partyID
 
+	logger.Info("SubscribeToSessionMessages called",
+		zap.String("session_id", sessionID),
+		zap.String("party_id", partyID),
+		zap.String("key", key),
+		zap.Int("current_channel_count", len(a.sessionChannels)))
+
 	// Create channel if not exists
 	if _, exists := a.sessionChannels[key]; !exists {
 		a.sessionChannels[key] = make(chan *entities.MessageDTO, 100)
+		logger.Info("Created new session channel",
+			zap.String("key", key))
 	}
 
 	ch := a.sessionChannels[key]