FINERACT-2455: Rework WorkingCapitalLoanAddChargeBusinessEvent

[Cocoa-Puffs]

Description

Describe the changes made and why they were made. (Ignore if these details are present on the associated Apache Fineract JIRA ticket.)

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[galovics]

Verified the collapse is safe - WorkingCapitalLoanChargeBusinessEvent had exactly one subclass, no reflection-based lookup by the abstract type (the classgraph scan used for external event config explicitly filters out abstract classes), and no listener anywhere references the removed Pre event. No Liquibase migration needed since the concrete event's TYPE string is unchanged - correctly none added.

One correction on the reasoning from the earlier pass, for the record: the abstract class wasn't a leftover from some other sibling event being removed - git history shows it was introduced together with the single subclass and never had a second one, so this was premature abstraction from day one rather than cleanup after a removal. Doesn't change the verdict, just wanted to flag it so the "sibling removed elsewhere" framing doesn't stick around as the reason in anyone's memory.

One nit in the new test: FeignWorkingCapitalLoanAddChargeBusinessEventTest collects createdProductIds in a list but the @AfterAll only clears it, never actually deletes the products - every run leaks one. BatchWorkingCapitalLoanIntegrationTest has the cleanup pattern to copy from.

Recommendation: APPROVE