fix!: split modulestore's has_course(ignore_case=True) was not working (#38044)

BREAKING CHANGE: this forces course IDs in modulestore to be unique (case insensitive). This was always supposed to be the case, but it wasn't working properly on MySQL. Upgrading past this commit may cause a migration failure if you have conflicting course IDs - see the migration 0004 docstring for details.

xmodule/modulestore/split_mongo/mongo_connection.py

@@ -15,6 +15,9 @@ from zoneinfo import ZoneInfo
 
 from ccx_keys.locator import CCXLocator
 from django.core.cache import caches, InvalidCacheBackendError
+from django.db.models import F
+from django.db.models.functions import Lower
+from django.db.models.lookups import Exact
 from django.db.transaction import TransactionManagementError
 import pymongo
 # Import this just to export it
@@ -659,13 +662,15 @@ class DjangoFlexPersistenceBackend(MongoPersistenceBackend):
         # We never include the branch or the version in the course key in the SplitModulestoreCourseIndex table:
         key = key.for_branch(None).version_agnostic()
         if not ignore_case:
-            query = {"course_id": key}
+            query_expr = Exact(F("course_id"), str(key))
         else:
             # Case insensitive search is important when creating courses to reject course IDs that differ only by
             # capitalization.
-            query = {"course_id__iexact": key}
+            # WARNING: 'course_id__iexact=key' does not work on this table as it uses a case-sensitive collation.
+            # We need to use the following explicit lowercase comparison in order to correctly query:
+            query_expr = Exact(Lower("course_id"), str(key).lower())
         try:
-            return SplitModulestoreCourseIndex.objects.get(**query).as_v1_schema()
+            return SplitModulestoreCourseIndex.objects.get(query_expr).as_v1_schema()
         except SplitModulestoreCourseIndex.DoesNotExist:
             # The mongo implementation does not retrieve by string key; it retrieves by (org, course, run) tuple.
             # As a result, it will handle read requests for a CCX key like