fix: swev-id: django__django-17087 use qualname for nested defaults

[casey-brooks]

Summary

• use klass.qualname when serializing bound class methods so nested classes resolve correctly
• add regression coverage for a nested TextChoices default on WriterTests

Testing

• PYTHONPATH=$PWD/tests:$PWD DJANGO_SETTINGS_MODULE=tests.test_sqlite .venv/bin/python tests/runtests.py migrations.test_writer.WriterTests.test_serialize_nested_class_method_default --parallel=1
• .venv/bin/flake8 django/db/migrations/serializer.py tests/migrations/test_writer.py

Reproduction Steps

1. On django__django-17087, run:

   PYTHONPATH=$PWD/tests:$PWD DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py migrations.test_writer.WriterTests.test_serialize_nested_class_method_default --parallel=1
   

2. Observed failure:

   AssertionError: "models.CharField(choices=[('A', 'A value'), ('B', 'B value')], default=migrations.test_writer.NestedChoicesWithDefault.default, max_length=1)" != "models.CharField(choices=[('A', 'A value'), ('B', 'B value')], default=migrations.test_writer.WriterTests.NestedChoicesWithDefault.default, max_length=1)"
   

Fixes #536}

[casey-brooks]

Test & Lint Summary

• PYTHONPATH=$PWD/tests:$PWD DJANGO_SETTINGS_MODULE=tests.test_sqlite .venv/bin/python tests/runtests.py migrations.test_writer.WriterTests.test_serialize_nested_class_method_default --parallel=1 (1 test, 1 passed)
• .venv/bin/flake8 django/db/migrations/serializer.py tests/migrations/test_writer.py (no issues)

[noa-lucent]

Switching to qualname fixes the nested TextChoices default regression. Added test covers the case.

[noa-lucent]

Switching to qualname fixes the missing enclosing class segment for nested TextChoices defaults. Regression test fails without the change and locks in the serialization string. I don't see other regressions.

[casey-brooks]

Local verification (Python 3.12.12):

• Tests: PYTHONPATH=$PWD/tests:$PWD DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py migrations.test_writer.WriterTests.test_serialize_nested_class_method_default --parallel=1 → 1/1 passed.
• Lint: flake8 django/db/migrations/serializer.py tests/migrations/test_writer.py → clean. (Ran via a small wrapper that patches
  importlib.metadata.EntryPoints.get so flake8 3.9.2 works on Python 3.12.)

[noa-lucent]

Looks good. Thanks!