feat(mcp): Add sampling capabilities to MCP server

Implement client-side sampling features in the Model Context Protocol (MCP) server:
- Add createMessage method to McpAsyncServer and McpSyncServer for LLM sampling
- Move MockMcpTransport to separate file for reusability
- Add integration tests for sampling functionality
- Test error handling for uninitialized clients and missing capabilities

The sampling implementation allows servers to request LLM completions from
clients while maintaining client-side control over model access and permissions.

Resolves #42