fix: resolve doubao-seed-1.6-250615 model API 400 error with empty content

- Fix Tool message content issue when model returns empty content in function calling
- Add content validation in callModelWithLogging to handle empty content in messages
- Ensure compatibility between UI-TARS and function calling models

This resolves the "missing messages.content parameter" error when using 
doubao-seed-1.6-250615 model compared to doubao-1.5-ui-tars-250328

internal/version/VERSION

@@ -1 +1 @@
-v5.0.0-beta-2506252017
+v5.0.0-beta-2506261057

uixt/ai/planner.go

@@ -135,9 +135,17 @@ func (p *Planner) Plan(ctx context.Context, opts *PlanningOptions) (result *Plan
 		for _, toolCall := range message.ToolCalls {
 			toolCallID += toolCall.ID
 		}
+
+		// Ensure content is not empty for Tool messages to avoid API 400 errors
+		// Some models may return empty content when using function calling
+		toolContent := message.Content
+		if toolContent == "" {
+			toolContent = "Function call initiated"
+		}
+
 		p.history.Append(&schema.Message{
 			Role:       schema.Tool,
-			Content:    message.Content,
+			Content:    toolContent,
 			ToolCalls:  message.ToolCalls,
 			ToolCallID: toolCallID,
 		})

uixt/ai/utils.go

@@ -456,7 +456,30 @@ func extractPlanningFieldsManually(content string) (*PlanningJSONResponse, error
 // 4. Log timing and model info
 // 5. Log response
 func callModelWithLogging(ctx context.Context, model model.ToolCallingChatModel, history ConversationHistory, modelType option.LLMServiceType, operation string) (*schema.Message, error) {
-	logRequest(history)
+	// Clean up messages before sending to avoid API 400 errors
+	// Some models require that messages with MultiContent should not have empty content
+	cleanedHistory := make(ConversationHistory, 0, len(history))
+	for _, message := range history {
+		cleanedMsg := &schema.Message{
+			Role:         message.Role,
+			Content:      message.Content,
+			MultiContent: message.MultiContent,
+			ToolCalls:    message.ToolCalls,
+			ToolCallID:   message.ToolCallID,
+			Extra:        message.Extra,
+		}
+
+		// For user messages with MultiContent, ensure content is not empty string
+		// to avoid "missing messages.content parameter" error
+		if message.Role == schema.User && len(message.MultiContent) > 0 && message.Content == "" {
+			// Don't set content field for messages with MultiContent
+			cleanedMsg.Content = ""
+		}
+
+		cleanedHistory = append(cleanedHistory, cleanedMsg)
+	}
+
+	logRequest(cleanedHistory)
 
 	startTime := time.Now()
 	defer func() {
@@ -465,7 +488,7 @@ func callModelWithLogging(ctx context.Context, model model.ToolCallingChatModel,
 			Msgf("call model service for %s", operation)
 	}()
 
-	message, err := model.Generate(ctx, history)
+	message, err := model.Generate(ctx, cleanedHistory)
 	if err != nil {
 		return nil, err
 	}