Assign students to groups in bulk. Each assignment maps one group (by ID or external reference) to multiple students (by IDs or external references). Returns 200 when all items succeed, 207 for partial success with per-item failures.

Per-item error codes: VALIDATION_ERROR, MISSING_GROUP_IDENTIFIER, AMBIGUOUS_GROUP_IDENTIFIER, MISSING_STUDENT_IDENTIFIER, AMBIGUOUS_STUDENT_IDENTIFIER, GROUP_NOT_FOUND, STUDENT_NOT_FOUND, STUDENT_ARCHIVED, ASSIGN_FAILED.