fix(windows): add windowsHide to detached spawn calls to suppress console windows (#44693)

The restart helper and taskkill spawn calls were missing windowsHide: true,
causing visible command prompt windows to flash on screen during gateway
restart and process cleanup on Windows.

src/process/kill-tree.ts

@@ -83,6 +83,7 @@ function runTaskkill(args: string[]): void {
     spawn("taskkill", args, {
       stdio: "ignore",
       detached: true,
+      windowsHide: true,
     });
   } catch {
     // Ignore taskkill spawn failures