Implements concrete StdioMCPTransport class that:
- Spawns MCP server as subprocess and communicates via JSON-RPC 2.0 over stdio
- Performs MCP handshake (initialize + notifications/initialized)
- Supports tools/list and tools/call methods
- Uses RLock for thread-safe concurrent access
- Auto-selected when transport='stdio' in MCPServerConfig
Adds BDD tests for stdio transport covering:
- Connection lifecycle and error handling
- Tool discovery and invocation
- MCPToolAdapter integration
ISSUES CLOSED: #4918
CoreRasurae