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

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,
 		})