Assign students and/or groups to a single course, identified by its external reference ID. Uses replace semantics: provided student/group lists replace existing enrollments. Omitted fields remain unchanged.

XOR rules:

• studentIds XOR studentExternalReferenceIds (optional)
• groupIds XOR groupExternalReferenceIds (optional)
• At least one of students or groups must be provided

Error codes: AMBIGUOUS_STUDENT_IDENTIFIER, AMBIGUOUS_GROUP_IDENTIFIER, MISSING_ENROLLMENT_DATA, COURSE_NOT_FOUND, ARCHIVED_COURSE_EXISTS, STUDENTS_NOT_FOUND, ARCHIVED_STUDENT_EXISTS, GROUPS_NOT_FOUND, ARCHIVED_GROUP_EXISTS, MAX_STUDENTS_EXCEEDED.

Rate limit: HEAVY. Idempotency: 5s deduplication window.