fix: correct course_id type in enrollment retrieval

[Ang-m4]

Description

This pull request makes a minor adjustment to the way course IDs are handled when fetching enrollments. Specifically, it ensures that the course_id parameter is always passed as a string to the get_enrollment function, which can help prevent potential type-related issues.

• Always convert course_id to a string when calling get_enrollment in has_an_edx_platform_enrollment to ensure consistent parameter types.