✨ Add simple_generalist A2A agent for testing with AppWorld MCP

[kellyaa]

Summary

This PR adds a new a2a/simple_generalist example agent that exposes an A2A-compatible server backed by AG2, with optional MCP tool usage.

Key changes:

• Add new a2a/simple_generalist project (packaging, lockfile, Dockerfile, env template, README).
• Implement A2A server wiring in a2a/simple_generalist/src/simple_generalist/a2a_server/server.py:
  • Agent card + skill metadata
  • Request handling + streaming task updates/events
  • Per-request MCP connection via streamablehttp_client when MCP_SERVER_URL is configured
• Implement AG2-driven generalist agent in a2a/simple_generalist/src/simple_generalist/agent/generalist_agent.py:
  • ConversableAgent + UserProxyAgent execution loop (a_initiate_chat)
  • Optional MCP toolkit registration for tool calling
  • Final answer extraction from AG2 chat history
• Add environment-driven settings in a2a/simple_generalist/src/simple_generalist/config/settings.py:
  • LLM model/key/base URL/extra headers
  • max iterations
  • optional public A2A URL
• Add runtime entrypoint in a2a/simple_generalist/src/simple_generalist/main.py.
• Add setup/documentation notes for local and Kagenti usage in a2a/simple_generalist/README.md.

Related issue(s)

kagenti/kagenti#543