FINERACT-2486: Fix guarantor endpoint returning savingsAccount.id=0 f…

[rhopman]

…or group savings

Description

When retrieving guarantor details via GET /loans/{loanId}/guarantors, the response shows savingsAccount.id = 0 for guarantors backed by group savings accounts, even though the on-hold transactions are correctly recorded with the actual savings account ID.

Checklist

• Write the commit message as per our guidelines
• Acknowledge that we will not review PRs that are not passing the build ("green") - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
• Create/update unit or integration tests for verifying the changes made.
• Follow our coding conventions.
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
• This PR must not be a "code dump". Large changes can be made in a branch, with assistance. Ask for help on the developer mailing list.

Your assigned reviewer(s) will follow our guidelines for code reviews.

[IOhacker]

Lgtm