fix Unable to see generic attributes on abstract classes #4058

[asukaminato0721]

Summary

Fixes #4058

generic class attributes remain visible when accessed through type[Self@...], which is the cls receiver in classmethods.

Bare class access like C.x still keeps the existing missing-attribute behavior.

Test Plan

add test

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- ERROR steam/ext/commands/converters.py:239:13-30: Generic attribute `converter_for` of class `Converter` is not visible on the class [no-access]

prefect (https://github.com/PrefectHQ/prefect)
- ERROR src/prefect/workers/base.py:753:12-43: Generic attribute `job_configuration_variables` of class `BaseWorker` is not visible on the class [missing-attribute]
- ERROR src/prefect/workers/base.py:754:22-43: Generic attribute `job_configuration` of class `BaseWorker` is not visible on the class [missing-attribute]
- ERROR src/prefect/workers/base.py:762:32-81: Object of class `object` has no attribute `model_json_schema` [missing-attribute]
- ERROR src/prefect/workers/base.py:765:34-55: Generic attribute `job_configuration` of class `BaseWorker` is not visible on the class [missing-attribute]

static-frame (https://github.com/static-frame/static-frame)
- ERROR static_frame/core/store_zip.py:386:16-39: Generic attribute `_STORE_CONFIG_CLASS` of class `_StoreZipDelimited` is not visible on the class [missing-attribute]

artigraph (https://github.com/artigraph/artigraph)
- ERROR src/arti/internal/mappings.py:246:53-72: Generic attribute `__target_type__` of class `TypedBox` is not visible on the class [missing-attribute]
- ERROR src/arti/internal/mappings.py:253:64-83: Generic attribute `__target_type__` of class `TypedBox` is not visible on the class [missing-attribute]
- ERROR src/arti/storage/__init__.py:90:9-40: Generic attribute `storage_partition_type` of class `Storage` is not visible on the class [no-access]
- ERROR src/arti/storage/__init__.py:105:31-57: Generic attribute `storage_partition_type` of class `Storage` is not visible on the class [missing-attribute]
- ERROR src/arti/storage/__init__.py:115:53-79: Generic attribute `storage_partition_type` of class `Storage` is not visible on the class [missing-attribute]