add example of gradio in streaming agent with mcp cases

[Sucran]

add example of gradio in streaming agent with mcp cases for issue #862

see issue #862

This PR commit an example which shows a best practice about using gradio and agents sdk to build a streaming app with mcp tools.

Edit points

• event_handler.py : an chain of responsibility implemention to process all stream events defined in agents sdk
• main.py : an implementation of gradio app and two agents based on deepseek-v3 and gradio mcp tool
• README.md and demo.png: show how this examples works.
• pyproject.toml : add gradio into dev dependency list and gradio 5.x version need ruff>=0.9.3 and python >= 3.10.
• uv.lock: sync change to pyproject.toml

Features

1. deepseek model integration examples
2. gradio ChatInterface examples with tool and agent Integrations
3. chain of responsibility implemention examples to process all stream events defined in agents sdk

[Sucran]

@rm-openai hi, Can you please help me review the code? Thank you very much

[Sucran]

@rm-openai hi，the conflict is solved. I am thinking, is it better to integrate Gradio in the Extension way mentioned in #855? would love to know your opinion.
Then I have considered using RunHooks and AgentHooks to implement it, but I found that it does not support inference events. Do you think Hooks would be better?