chore: Allow OS to assign port for unit tests

[keelerm84]

Previously we started looking for testing ports starting at 5000. If the
port was in use, we would incrementally search upwards until we found a
port we could use.

A recent change in the GH Windows image resulted in a permission error
being raised before we could find a port to use:

Failure/Error: WEBrick::HTTPServer.new(base_opts)

Errno::EACCES:
Permission denied - bind(2) for 127.0.0.1:50018

To address this, we are modifying the tests to set a port of 0, which
will allow the OS to assign an available port for us. This removes the
need for this silly retry logic and hard coded port numbers.

---

Note

Switch StubHTTPServer to OS-assigned ephemeral ports and set CI matrix fail-fast to false.

• Tests:
  • Use OS-assigned ephemeral port by setting Port: 0 in spec/http_util.rb and capturing actual port from server.config[:Port].
  • Remove manual port allocation, retry logic, and @@next_port/self.next_port helpers; simplify create_server signature.
• CI:
  • Disable matrix fail-fast in jobs.build-linux.strategy of .github/workflows/ci.yml.

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