Fix IntervalStrategy#decrement in ActiveSupport 7.1

[meqif]

Context

ActiveSupport 7.1 introduced a breaking change in ActiveRecord::Cache#decrement's behavior:

• prior to 7.1, decrement on a new key does nothing and returns nil
• since 7.1, decrement on a new key sets its value to 0 first and then decrements, returning a negative value

The new behavior is at odds with Prop::IntervalStrategy#decrement's intended behavior, which sets new keys to 0 when they are decremented.

Changes

This PR fixes IntervalStrategy#decrement by detecting if the return value of ActiveRecord::Cache#decrement is nil (< 7.1) or -amount (>= 7.1) before clamping the value to 0. This is as vulnerable to a race condition as the existing code, so it's not worse in that regard.

Prop::IntervalStrategy#increment does not need a similar change because incrementing a new key is the same as incrementing an existing key set to 0 in ActiveSupport 7.1, just like we intended, and thus the fallback to write isn't used in that case.

It also adds some missing require statements to the test helper: the time-related ActiveSupport extensions (e.g. .minutes) are no longer automatically loaded since Rails 7.1. In turn, active_support/core_ext/numeric/time relies on new deprecation-related functionality that isn't automatically loaded either, so we try to load that as well.