fix: close ProxyAgent and Agent dispatchers in test to prevent afterAll timeout

In undici v8, ProxyAgent and Agent maintain persistent connections (especially
H2 connections) that keep the proxy server's CONNECT tunnels alive. Close all
dispatcher instances at the end of each test to allow clean proxy server
shutdown within the afterAll timeout.

Agent-Logs-Url: https://github.com/node-modules/urllib/sessions/bcba591d-864f-4402-8861-c5be26f032ba

Co-authored-by: fengmk2 <156269+fengmk2@users.noreply.github.com>

Author: Copilot

test/options.dispatcher.test.ts

@@ -51,6 +51,7 @@ describe('options.dispatcher.test.ts', () => {
     // console.log(response2.status, response2.headers);
     assert.equal(response2.status, 200);
     assert.equal(response2.data.name, 'urllib');
+    await proxyAgent.close();
   });
 
   it('should work with getGlobalDispatcher() dispatcher', async () => {
@@ -64,6 +65,7 @@ describe('options.dispatcher.test.ts', () => {
     assert.equal(response.status, 200);
     assert.equal(response.data, '<h1>hello</h1>');
     setGlobalDispatcher(agent);
+    await proxyAgent.close();
   });
 
   it('should work with http/2 dispatcher', async () => {
@@ -81,5 +83,6 @@ describe('options.dispatcher.test.ts', () => {
     });
     assert.equal(response.status, 200);
     assert.match(response.headers['content-type'] ?? '', /application\/json/);
+    await agent.close();
   });
 });