AI Tiered Plans: Address 0 count when the site does not have a plan

For Simple sites with upgrades, the current usage counter will return 0 because the site does not have a Jetpack AI plan, and therefore does not have a plan "anniversary" day each month, so the code will return 0 in this case.

Is it relevant to deal with Simple sites in this context? How could we do it?

One idea is to use the WPCOM upgrade plan date as the date of the renewal, since the Jetpack AI is being offered by this specific plan.

Trace the current usage counter and the logic that derives the Jetpack AI plan anniversary day. Compare that behavior for Simple sites with the WPCOM upgrade plan date proposed in the issue; done means the counter no longer returns 0 when the site lacks a Jetpack AI plan and its renewal basis is defined.

Written by the indexing model from the issue text.