fix(copilot): serialize tool output to JSON in ToolMessage for history

Fixed tool output serialization in tool_node function to ensure
ToolMessage.content is always in JSON format, not Python str() representation.

This fixes the issue where conversation history showed tool outputs
with single quotes (Python format) instead of standard JSON.

Changes:
- Added json import to gns3_copilot.py
- Modified tool_node() to serialize observation to JSON before creating ToolMessage
- Ensures both SSE streaming and history storage use consistent JSON format

Root cause: ToolMessage was created with raw dict/list objects, which
LangChain converted to Python str() representation when saving to history.

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

gns3server/agent/gns3_copilot/agent/gns3_copilot.py

@@ -45,6 +45,7 @@ Copilot Modes:
 """
 
 # Standard library imports
+import json
 import logging
 import operator
 from datetime import datetime
@@ -384,6 +385,11 @@ def tool_node(state: dict, config: RunnableConfig | None = None):
             logger.error("Tool %s failed: %s", tool_name, e, exc_info=True)
             observation = f"Error: {str(e)}"
 
+        # Serialize observation to JSON string if it's not already a string
+        # This ensures ToolMessage.content is always JSON format, not Python str()
+        if not isinstance(observation, str):
+            observation = json.dumps(observation, ensure_ascii=False, indent=2)
+
         # Create ToolMessage with metadata including created_at
         tool_msg = ToolMessage(
             content=observation,