FINERACT-415: Enforce exact math validation for loan product mandatory guarantee

[Bhavya-Sonigra]

Closes FINERACT-415

Description

Updated validateGuaranteeParams in LoanProductDataValidator.java to enforce strict equality (!= 0) instead of a loose less-than check (< 0).

Previously, the Fineract API allowed loan product creation if the mandatory guarantee was strictly greater than the sum of the own funds and guarantor funds, leaving the remaining balance completely unaccounted for in the database. This patch ensures the API strictly validates that the mandatoryGuarantee is exactly equal to the sum of minimumGuaranteeFromOwnFunds and minimumGuaranteeFromGuarantor (or exactly equals one of them if the other is null/zero) before allowing creation or updates. Payload rejections now return the must.be.equal.to.sum.of.own.and.guarantor.funds error code.

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