FINERACT-GSOC: Modernize UserHelper and RolesHelper with Fineract-Client

[Avtansh88]

## Description

This PR is a Proof of Concept (PoC) for my GSoC 2026 proposal: "Modernizing Integration Tests with Fineract-Client".

Key Changes:

->UserHelper.java: Refactored legacy RestAssured methods to use type-safe Fineract-Client (Feign).

->RolesHelper.java: Introduced a modern, parameter-less createRole() method to resolve inter-helper dependencies and ensure backward compatibility.

->Type Safety: Replaced manual JSON string building with OpenAPI models like PostUsersRequest and PutUsersUserIdRequest.

->Code Cleanup: Removed redundant RequestSpecification and ResponseSpecification from modernized method signatures.

Checklist

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