Why this change?
We have been debugging flaky system tests and noticed in https://github.com/discourse/discourse/actions/runs/7911902047/job/21596791343?pr=25690
that ActiveRecord connection checkout timeouts are encountered because
the Capybara server thread is processing requests even after
`Capybara.reset_session!` and ActiveRecord's `teardown_fixtures` have already been call.
The theory here is that an inflight request can still hit the Capybara
server even after `Capybara.reset_session!` has been called and end up
eating up an ActiveRecord connection for too long and also messing with
the database outside of a transaction.
What does this change do?
This change adds a `BlockRequestsMiddleware` middleware in the test
environment which is enabled to reject all incoming requests at the end
of each system test and before `Capybara.reset_session!` is called. At
the start of each RSpec test, the middleware is disabled again.
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