Stabilize test server boot on macOS

[tagliala]

Ensure the spec helper's WEBrick server starts and is detected reliably on recent macOS:

• Bind WEBrick explicitly to localhost and probe readiness via 127.0.0.1 to avoid IPv6/IPv4 localhost resolution mismatches that triggered Net::OpenTimeout during boot. (Copilot)
• Use a single Rackup/Rack WEBrick handler constant consistently
• Keep WEBrick for tests since it supports custom methods (e.g. PURGE) that stricter servers reject.
• Trap INT and call handler.shutdown via the shared constant to cleanly stop the server process, helping prevent orphaned servers and file descriptor leaks (seen as Errno::EMFILE after reruns).
• Replace deprecated top-level TimeoutError with Timeout::Error and require timeout

With these changes, the test server comes up promptly and the suite no longer times out on Apple Silicon/macOS, and repeated runs don't exhaust file descriptors.

Fix #260

[tagliala]

This is co-authored by Copilot, actually it did find the issue with IPv4 IPv6 which was preventing the server to work on macos

Failures are unrelated (segfault)

[felipedmesquita]

Thanks, @tagliala!
On my Ventura Intel Mac, the test still fail in this branch. It also seems we've accidentally found the first consistent CI segfault here with ruby 2.6, as I've ran the test 7 times without success.
I don't have an apple silicon mac to test this today, but since this fixes the failures for you, I'm merging it in. It's better to have the tests working for newer macs than have 2.6 passing in CI for now.