Support mcp prompts

[thoo]

Summary

Add MCP prompt support to enable user-controlled agent instruction generation. This enhancement allows MCP servers to provide prompts that can be used to dynamically generate agent instructions, enabling more flexible and user-controlled agent behavior.

Key changes:

• Added abstract list_prompts() and get_prompt() methods to base MCPServer class
• Implemented prompt support in MCPServerStdio and MCPServerStreamableHttp classes
• Added comprehensive test suite for prompt functionality
• Created example MCP prompt server with working demonstration
• Updated documentation with prompt usage examples

Note: This PR implements MCP prompt support only. Resource support is not included in this implementation.

Test plan

• Unit tests: Added 11 comprehensive tests in tests/mcp/test_prompt_server.py covering:
  • Prompt listing and retrieval
  • Argument formatting and validation
  • Agent integration with prompt-generated instructions
  • Streaming and non-streaming scenarios
  • Error handling for missing prompts
• Integration tests: Updated existing MCP test suite to handle new abstract methods
• Example verification: Created working example with MCP prompt server and client
• All tests pass: 450/450 tests passing after adding optional dependencies

Issue number

Partially addresses #544 (prompts only, resources not implemented)

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass