fix: update program enrollments to be case insensitive on external_user_key (#29646)

UTAsutin is an example partner who would use mixed casing on their external_user_key references for program enrollment upload and matriculation. Update the system to be case insensitive on external_user_key

Co-authored-by: Simon Chen <schen@edX-C02FW0GUML85.local>

lms/djangoapps/program_enrollments/api/linking.py

@@ -11,6 +11,7 @@ import logging
 
 from django.contrib.auth import get_user_model
 from django.db import IntegrityError, transaction
+from requests.structures import CaseInsensitiveDict
 
 from common.djangoapps.course_modes.models import CourseMode
 from common.djangoapps.student.api import get_access_role_by_role_name
@@ -158,10 +159,10 @@ def _get_program_enrollments_by_ext_key(program_uuid, external_user_keys):
     ).prefetch_related(
         'program_course_enrollments'
     ).select_related('user')
-    return {
+    return CaseInsensitiveDict({
         program_enrollment.external_user_key: program_enrollment
         for program_enrollment in program_enrollments
-    }
+    })
 
 
 def _get_lms_users(lms_usernames):