[28.x] Subscription Billing] Infinite loop in CalculateBillingPeriod when HarmonizeNextBillingTo caps at stale "Next Billing To" date

[sanjmaurya]

Bug 637042: [ALL-E][Subscription Billing] Infinite loop in CalculateBillingPeriod when HarmonizeNextBillingTo caps at stale "Next Billing To" date

Fixes AB#637042

Issue:- CalculateBillingPeriod in BillingProposal.Codeunit.al enters an infinite loop (hang / no response) when generating a billing proposal for a harmonized customer subscription contract whose "Next Billing To" date is stale (older than the current billing period being calculated).
Cause :- Root Cause
The while loop advances the billing period end date by repeatedly calling CalculateNextBillingToDateForServiceCommitment:

For a harmonized contract, CalculateNextBillingToDateForServiceCommitment calls HarmonizeNextBillingTo, which caps the computed date at CustomerContract."Next Billing To":

When the contract's "Next Billing To" is a stale date earlier than BillingDate, every iteration gets capped back to that same stale value. So BillingPeriodEnd never advances past it, the loop condition BillingPeriodEnd < BillingDate stays true forever, and the loop never terminates.
Solution
Add a progress guard that breaks the loop as soon as BillingPeriodEnd stops advancing (i.e., is capped to the same/earlier date). A PreviousBillingPeriodEnd tracker is introduced, and the loop only continues while BillingPeriodEnd > PreviousBillingPeriodEnd: