fix: return certificate_available_date in courses list API response

[Anas12091101]

Description

The /api/courses/v1/courses/ list endpoint was not returning certificate_available_date even when set in Studio, because the field was only serialized in CourseDetailSerializer (detail endpoint).

This PR adds to_representation() to CourseSerializer so the field is included in list responses under the same condition: the certificates.auto_certificate_generation waffle switch is enabled and the course is instructor-paced.

Also, this PR adds unit tests covering both the included and excluded cases.

Useful information to include:

• Which edX user roles will this change impact? "Learner", "Course Author"
• Include screenshots for changes to the UI (ideally, both "before" and "after" screenshots, if applicable).
• Provide links to the description of corresponding configuration changes. Remember to correctly annotate these
  changes.

Supporting information

https://github.com/mitodl/hq/issues/11231

Testing instructions

Prerequisites: A running Open edX LMS with a course that has:

• Pacing: Instructor-paced
• Certificate display behavior: end_with_date
• A certificate_available_date set

1. Enable the waffle switch

In Django admin (/admin/waffle/switch/), ensure certificates.auto_certificate_generation is active. Without it, the field is never included regardless of this change.

2. Hit the list endpoint

curl -H "Authorization: Bearer <token>" \
  "http://localhost:8000/api/courses/v1/courses/?course_keys=course-v1:OpenedX+DemoX+DemoCourse"

3. Verify the response contains the field

{
  "results": [
    {
      "id": "course-v1:OpenedX+DemoX+DemoCourse",
      ...
      "certificate_available_date": "2026-05-27T00:00:00Z"
    }
  ]
}

4. Confirm the detail endpoint still works

curl -H "Authorization: Bearer <token>" \
  "http://localhost:8000/api/courses/v1/courses/course-v1:OpenedX+DemoX+DemoCourse/"

Same field should appear there too (no regression).

5. Negative case — disable the waffle switch and confirm certificate_available_date is absent from both responses.

Deadline

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

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.
• If your database migration can't be rolled back easily.

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