fix: optimize enrollment counts to use read replica and show all configured modes

[brianjbuck-wgu]

Description

Optimizes the enrollment data in the Instructor API v2 Course Info endpoint (CourseInformationSerializerV2) to fix three issues:

1. Show all configured modes: Enrollment counts now include all modes configured for the course via CourseMode.modes_for_course(), even those with 0 enrollments. Modes not configured for the course are omitted. Previously, only modes with at least 1 enrollment appeared in the response.

2. Reduce redundant DB queries: Replaced 3 separate database queries with a single call to the existing CourseEnrollment.objects.enrollment_counts() manager method.

3. Use read replica: The manager method routes queries through use_read_replica_if_available(), reducing load on the primary database.

Impacted roles: Course Author, Operator (Instructor Dashboard MFE consumers)

Files changed:

• lms/djangoapps/instructor/views/serializers_v2.py — Refactored get_enrollment_counts() and get_total_enrollment()
• lms/djangoapps/instructor/tests/test_api_v2.py — Updated and added enrollment count tests
• lms/djangoapps/instructor/docs/references/instructor-v2-course-info-spec.yaml — Updated schema to document dynamic mode keys

Supporting information

• Fixes: Instructor Dashboard - Update enrollment data in Instructor API v2 Course Info endpoint #38006
• Slack discussion on enrollment mode display

Other information

• No database migrations required
• No dependencies on other changes
• Backward compatible — the response shape is the same (object with mode keys + total), but now includes additional keys for configured modes that previously had 0 enrollments

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