feat: add hasUnmetPrerequisitesList in CoursewareAccessSerializer

[jignaciopm]

Description

This pull request enhances the prerequisite course serialization in the learner home API by adding a new field that provides detailed information about unmet prerequisites. It introduces a new serializer method to return a list of prerequisite courses not yet completed, including their display names and URLs, and adds corresponding unit tests to ensure correct behavior.

Prerequisite course serialization improvements:

• Added a new serializer field hasUnmetPrerequisitesList to CoursewareAccessSerializer, which returns detailed info for each unmet prerequisite course, including display name, course home URL, and about page URL. [1] [2]
• Implemented the helper method _get_course_about_url_for_key to reliably generate the course about page URL, handling cases where the course overview does not exist.

Testing enhancements:

• Added unit tests to verify that hasUnmetPrerequisitesList returns an empty list when there are no unmet prerequisites and correctly serializes prerequisite course info when present.

Dependency and import updates:

• Imported new helper functions and models (get_link_for_about_page, get_pre_requisite_courses_not_completed, CourseOverview) to support the new functionality.

Supporting information

https://github.com/openedx/frontend-app-learner-dashboard/issues/753

Testing instructions

• Create a simple course in Studio.
• Go to "Advanced settings" and set "Enable subsection prerequisites" ON (true).
• Go to "Schedule & details" and set in "Prerequisite course" whatever you want.
• Check the /api/learner_home/init endpoint. Locate your created course and find the attribute hasUnmetPrerequisitesList. You will see the following information:

"hasUnmetPrerequisitesList": [
    {
        "display": "PruebaReq1 PruebaReq1",
        "url": "http://apps.local.openedx.io:2000/learning/course/course-v1:PruebaReq1+PruebaReq1+PruebaReq1/home",
        "about_url": "http://local.openedx.io:8000/courses/course-v1:PruebaReq1+PruebaReq1+PruebaReq1/about"
    },
    {
        "display": "Another Course Another Org",
        "about_url": "https://google.com/"
    }
]

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.