FINERACT-2048: Make amount/recurrence fields optional for DUES standing instruction type

[Monica-CodingWorld]

Description

Modified StandingInstructionDataValidator.java to make amount, interval, recurrence frequency, and on month day fields optional when standing instruction type is "DUES". These fields are automatically picked from the loan repayment schedule for DUES type instructions, so they should not be required during validation.

Changes:

• CREATE validation: Made amount field .notNull() only for FIXED type; recurrence fields optional for DUES type
• UPDATE validation: Applied same conditional logic for partial updates; fixed variable scope for standingInstructionType
• Periodic validation: Moved recurrenceInterval.integerGreaterThanZero() inside if (isPeriodic) block where it belongs

JIRA ticket : https://issues.apache.org/jira/browse/FINERACT-2048

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[Aman-Mittal]

standingInstructionType == null || !StandingInstructionType.fromInt(standingInstructionType).isDuesAmoutTransfer()

You are using this logic in repeatedly throughout the code change i think you can extract this into a helper function

[Aman-Mittal]

This seems a little bit complicated. and if conditions can be merged, also of there is any business rule that is implemented make sure to document that helper function

Business rules should be explicit and self-documenting

while the Github Action is not run here is a pro tip you can run the check on your forked repo and add testcase for more coverage

[Monica-CodingWorld]

@Aman-Mittal Thank you so much for your guidance and suggestions. I will improve the PR and update shortly.