feat: implement standard MCP protocol with SSE transport

- Use FastMCP (Anthropic MCP SDK) for tool registration and SSE transport
- Mount SSE app under /v3/mcp/transport with JWT token authentication
- Token passed via ?token=<jwt> query parameter on SSE connection
- Token validated against GNS3 auth_service and stored in contextvars
- Tool handlers create Gns3Connector with JWT token to call GNS3 REST API
- 7 project tools: list_projects, get_project, create_project, delete_project,
  open_project, close_project, get_project_stats
- Unauthenticated SSE connections return 401

gns3server/api/server.py

@@ -83,6 +83,9 @@ def get_application() -> FastAPI:
 
 app = get_application()
 
+# Register MCP SSE transport routes (Starlette-level, for raw ASGI access)
+mcp.register_starlette_routes(app)
+
 # Monkey Patch uvicorn signal handler to detect the application is shutting down
 app.state.exiting = False
 unicorn_exit_handler = UvicornServer.handle_exit