use fallback_service instead of nest_service for MCP router (#8566)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

backend/windmill-api/src/mcp/core.rs

@@ -546,7 +546,7 @@ pub async fn setup_mcp_server(
     let service =
         StreamableHttpService::new(move || Ok(runner.clone()), session_manager, service_config);
 
-    let router = Router::new().nest_service("/", service);
+    let router = Router::new().fallback_service(service);
     Ok((router, cancellation_token))
 }