PAYMENTS-11727 Proof: the fork delivery specs fail on 0.8.3 as released

[WillemHoman]

Jira: PAYMENTS-11727

Not for merge. This exists to demonstrate that the regression specs added in
#45 would have caught the metric loss that 0.8.3 shipped.

What? Why?

main is 0.8.3, the release that dropped webhook metrics in bigpay. This branch adds nothing but the specs from #45,
cherry-picked, and the CI job that runs them.

The result is the completeness property failing and the overhead property passing, which is the signature of that
incident: job time stayed flat while the observations silently went nowhere.

1) metric delivery from Resque forked children completeness delivers one observation for every job that pushed one
   Failure/Error: expect(exporter.count_for(ForkDeliveryProbeJob::METRIC_NAME)).to eq JOB_COUNT

     expected: 100
          got: 0

2 examples, 1 failure

Zero of 100, not a partial loss. Production saw roughly 8% survive rather than 0% because a real webhook job does a
little work after its push and sometimes wins the race; the probe job pushes as its final statement, so it never does.

Commits

commit
209dd85	resque and sinatra >= 4.0 dev dependencies (cherry-pick)
48d0257	the completeness and overhead specs (cherry-pick)
1db4208	CI job on ruby 3.4 (cherry-pick)
db5261d	monotonic clock so specs load on ruby 4.0 (cherry-pick)
3505a25	one-line adaptation, see below

The adaptation is the only deviation from #45. The spec's setup calls client.reset_after_fork! to make the run
independent of spec ordering, and that method is part of the fix, so it does not exist here. Replaced with a direct
@queue.clear, which is the only part of it the setup needs. Nothing in either assertion changed.

How was it tested?

redis-server --port 6399 --save '' --appendonly no --daemonize yes
FORK_INTEGRATION=1 REDIS_URL=redis://127.0.0.1:6399/15 bundle exec rspec spec/integration

ruby-3_4-rspec_fork_integration is expected to be red on this branch. That is the point of it.

See also #44, which does the same for the PAYMENTS-11567
latency regression.