FINERACT-2892: Return Loan Product CREATE-check datatables in the template

[rymghosn]

Description

JIRA: FINERACT-2892. This is the backend half; showing the datatable on the Loan Product screen is web-app work.

Follow-up to #6538 (FINERACT-2874), which lets a Loan Product carry a CREATE Entity Data Table Check. This PR doesn't depend on #6538 to compile, but the gap only shows up once #6538 is in.

Root cause: the client, group, center, loan and savings templates all return datatables from EntityDatatableChecksReadService.retrieveTemplates(CREATE, <entity>, productId), so a UI knows which datatable entry the create form must collect. GET /loanproducts/template never did. With a Loan Product / CREATE check in place, POST /loanproducts then fails with 403 error.msg.entry.required.in.datatable, and the client has no way to discover which datatable it needs.

Fix: LoanProductData gets a settable datatables. LoanProductsApiResource.retrieveTemplate fills it from retrieveTemplates(CREATE, m_product_loan, null), the same way CentersApiResource does. datatables is also added to the response parameter set so fields= filtering keeps it.

Tests: new LoanProductsApiResourceTemplateDatatablesTest. One test checks that the CREATE-check datatables reach the serialized template and that datatables is in the parameter set. The other checks that the field is absent when no check exists.

Manual verification on a local build against PostgreSQL, with #6538 applied for both runs. The setup was a single-row datatable on m_product_loan plus an Entity Data Table Check (m_product_loan, status 100) on it:

• before: GET /loanproducts/template → 200 with no datatables key, and ?fields=id,datatables → 400 "The parameter datatables is not supported".
• after: datatables lists the datatable with its column headers, fields=id,datatables keeps it, and once the check is deleted the key is absent again.

Checklist

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• 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 (the template gains an optional datatables field shaped like the other templates'; the GetLoanProductsTemplateResponse Swagger model is not updated here)
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