Unit tests for paginated user_role_assignment endpoint

[craigyu]

Description
Write unit tests for #1626

Acceptance Criteria
the unit tests should test the following

• default pagination with no query params
• pagination with specific page number and page size
• sort by with default pagination and customized pagination params
• search with default pagination and customized pagination params
• Empty result should return []
• Isolated unit tests
• modify the endpoint to use updated_dated create_date as the default sorting field