Support mid-cycle add-on additions with immediate proration without resetting the subscription billing cycle

We need an option in the Change Plan API that allows us to add add-ons to an existing active subscription mid-cycle, immediately charge only the prorated amount for those newly added seats for the remaining days of the current cycle, without resetting or recalculating the overall subscription billing cycle.

Current Limitation:
As confirmed by Dodo Support (via Thambi), Currently, when trying to add a seat add-on mid-cycle:
If we use prorated_immediately, the system does not just isolate the new seat's prorated cost. Instead, it resets the entire subscription's billing cycle to the change date, recalculating the base plan charges from that day forward. This completely disrupts the customer's established renewal schedule.
If we use do_not_bill, the original renewal date is preserved, but we cannot collect payment for the new seats immediately; the charge is delayed entirely until the next renewal.

There is currently no way to charge immediately for just the added seat capacity while keeping the original billing cycle completely intact.