Test coverage and headless nvim bug fixes

nvim-core/src/lib.rs

@@ -726,6 +726,7 @@ pub async fn run_mcp_loop(app_name: &str, app_version: &str) {
             m
         } else {
             tracing::info!("Stdin closed, exiting loop");
+            headless::kill_headless_nvim().await;
             break;
         };