fix(copilot): serialize tool output to standard JSON format for frontend parsing

Changed tool output serialization in AgentService._convert_event_to_chunk()
from str() to json.dumps() to ensure structured data (dict/list) is properly
formatted as standard JSON instead of Python string representation.

Changes:
- Added json import to agent_service.py
- Modified on_tool_end event handling to use json.dumps(output, ensure_ascii=False, indent=2)
- Updated ai-chat-api-design.md to document tool_output format

Benefits:
- Frontend can parse tool results with standard JSON.parse()
- Chinese and non-ASCII characters are preserved (not escaped)
- Formatted output (indent=2) improves readability

Co-Authored-By: YueGuobin <yueguobin@outlook.com>

gns3server/agent/gns3_copilot/agent_service.py

@@ -32,6 +32,7 @@ in the project directory.
 """
 
 import asyncio
+import json
 import logging
 import os
 from datetime import datetime
@@ -460,9 +461,10 @@ class AgentService:
         elif event_type == "on_tool_end":
             # Tool execution completed
             output = data.get("output", "")
-            # Convert output to string if it's not already
+            # Convert output to JSON string if it's not already a string
+            # This ensures dict/list outputs are properly serialized for frontend parsing
             if not isinstance(output, str):
-                output = str(output)
+                output = json.dumps(output, ensure_ascii=False, indent=2)
             return {
                 "type": "tool_end",
                 "tool_name": event.get("name", ""),