fix(tool-web): bound HTML conversion work

examples/acp-agent/web-fetch-fixture-server.mjs

@@ -45,8 +45,11 @@ export async function apply(ctx) {
   })
   // The fixture must never hold the process open past protocol shutdown.
   server.unref()
-  ctx.effect(() => () => {
-    server.close()
-    server.closeAllConnections()
+  ctx.effect(() => async () => {
+    await new Promise((resolve, reject) => {
+      server.close(error => error ? reject(error) : resolve(undefined))
+      // Stop accepting first so a connection cannot arrive after the forced close.
+      server.closeAllConnections()
+    })
   }, 'web-fetch-fixture-server')
 }