Add error handler to third test's client script

Added req.on('error') handler to match the pattern used in the first test,
ensuring consistent error handling and debuggable failures.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/regression/issue/26638.test.ts

@@ -270,6 +270,11 @@ const req = https.request('https://localhost:${server.port}/', {
   });
 });
 
+req.on('error', (e) => {
+  console.error('Request error:', e.message);
+  process.exit(1);
+});
+
 // Write many chunks as fast as possible
 const chunk = '${chunkContent}';
 for (let i = 0; i < numChunks; i++) {