fix: prevent node-fetch test server close from hanging on Windows

[mcollina]

Description

The test/node-fetch/main.js suite was flaking on Windows with:

test at test\node-fetch\main.js:1:1
✖ D:\a\undici\undici\test\node-fetch\main.js (696.5575ms)
  'test failed'
Test results (58 passed, 1 failed)

This was caused by TestServer.stop() calling server.close() and awaiting the 'close' event, but on Windows, lingering socket connections could prevent the 'close' event from firing. The after hook would hang until the test runner's module-level timeout killed it, causing the entire test module to fail.

Fix

Call server.closeAllConnections() before server.close() in TestServer.stop() to ensure all active connections are destroyed before attempting to close the server. This is a standard pattern for test server teardown and is safe because closeAllConnections() was added in Node.js 18.2.0 and the project already requires Node.js 18+.

Testing

• Full npm run test:node-fetch passes (176 pass, 13 skipped, 0 fail)
• Fix addresses the root cause of the Windows CI timeout

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (4f8f814) to head (b44e031).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5246   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         110      110           
  Lines       36366    36366           
=======================================
  Hits        33922    33922           
  Misses       2444     2444           

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[trivikr]

Fixes: #5178