FINERACT-2550: Fix unsupported templateType returning null response

[devi-pathak2263]

Description

Fixes an issue where unsupported templateType values in retrieveApprovalTemplate(...) returned a null/empty response instead of throwing an error.

Changes

• Added explicit validation for unsupported templateType
• Throws NotSupportedLoanTemplateTypeException for invalid values

Impact

• Prevents silent failures
• Aligns behavior with other template APIs
• Improves API consistency

Additional Notes

This change aligns with existing usage of NotSupportedLoanTemplateTypeException in other template-related methods within LoansApiResource and ensures consistent error handling across APIs..

JIRA: https://issues.apache.org/jira/browse/FINERACT-2550

Checklist

Please make sure these boxes are checked before submitting your pull request - thanks!

• 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.