Fix flaky http-reverse-proxy test by using OS-assigned ports

[gonzaloriestra]

WHY are these changes introduced?

There's a flaky test:

FAIL   @shopify/app  src/cli/utilities/app/http-reverse-proxy.test.ts > http-reverse-proxy for http > routes requests to the default target when no matching path is found
Error: Test timed out in 5000ms.
If this is a long-running test, pass a timeout value as the last argument or configure it globally with "testTimeout".
 ❯ src/cli/utilities/app/http-reverse-proxy.test.ts:28:3
     26|   })
     27| 
     28|   test('routes requests to the default target when no matching path is…
       |   ^
     29|     const response = await fetch(`${protocol}://localhost:${ports.prox…
     30|     await expect(response.text()).resolves.toBe('Response from target

Example: https://github.com/Shopify/cli/actions/runs/24026356767/job/70065586128

WHAT is this pull request doing?

Replace getAvailableTCPPort() with port 0 in listen() calls to eliminate the TOCTOU race condition where a port could be claimed by another process before the server binds to it.

When listen() fails silently (unhandled error), the promise never resolves and the test hangs until the 5000ms timeout fires.

Also simplifies the test fixtures by merging the ports and servers fixtures into a single setup fixture, since ports are now derived from the actual server addresses.

How to test your changes?

CI

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

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