fix below bson_obj document size error due to mongo aggregate function by calculating the count and using find instead of aggregate.
```
raise OperationFailure(errmsg, code, response, max_wire_version)
pymongo.errors.OperationFailure: BSONObj size: 19117457 (0x123B591) is invalid. Size must be between 0 and 16793600(16MB) First element: _id: null, full error: {'ok': 0.0, 'errmsg': 'BSONObj size: 19117457 (0x123B591) is invalid. Size must be between 0 and 16793600(16MB) First element: _id: null', 'code': 10334, 'codeName': 'BSONObjectTooLarge', '$clusterTime': {'clusterTime': Timestamp(1750073176, 2), 'signature': {'hash': b'\xd7\xdd\xf4\x7f\xe4\xcea\xa1\xa7P\xba\xab\xcb\x12\x7f1A$(\xb4', 'keyId': 7471973240714297345}}, 'operationTime': Timestamp(1750073176, 2)}
```
This is part of Old Mongo removal. Also in this commit:
* fixed 400 error for cms old_style assets
* fix TEST_DATA_SPLIT_MODULESTORE import for test_course_index
- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
- More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore
