Running with Rack > 3.1 gives error `uninitialized constant Rack::Handler`

[pezholio]

Pre issue-raising checklist

I have already:

• [x] Read through the relevant docs at https://docs.pact.io
• [x] Upgraded to the latest version of the gem
• [x] Checked the CHANGELOG to see if the issue I am about to raise has been fixed
• [x] Created an executable example that demonstrates the issue using either a:
  • Dockerfile
  • Git repository with a Travis or Appveyor (or similar) build

Software versions

• OS: Mac OSX 14.5
• pact mock service: v 3.12.1

Expected behaviour

Tests pass

Actual behaviour

When running the tests with Rack version >= 3.1.0, the tests fail with the error:

NameError:
    uninitialized constant Rack::Handler

This looks related to changes in Rack starting 3.1.0 - https://github.com/rack/rack/blob/main/CHANGELOG.md#310---2024-06-11

Steps to reproduce

An example build with Rack > 3.1.0 ❌

https://github.com/alphagov/pact-mock_service/actions/runs/9513302857

And example build with Rack < 3.1.0 ✅

https://github.com/alphagov/pact-mock_service/actions/runs/9513330459

[Harriethw]

I've put a fix that worked for me locally here https://github.com/pact-foundation/pact-mock_service/pull/146/files#r1639534752