YueGuobin b936a52f6f docs: add LangGraph Agent and tool output serialization documentation
Added new section in ai-chat-api-design.md explaining:

- LangGraph Agent architecture and main components
- tool_node function and tool output serialization mechanism
- Why serialization happens in tool_node (not agent_service)
- Tool output data flow diagram showing SSE and history paths
- Explanation of JSON format consistency across streaming and storage

This documentation clarifies the fix for single-quotes issue in
conversation history and helps future maintenance.

Co-Authored-By: YueGuobin <yueguobin@outlook.com>
2026-03-09 15:54:44 +08:00
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