Fix Config subclass attribute discovery and caching

[jmahotiedu]

Summary

• fix Config.attributes() to cache metadata per-class instead of inheriting another class's cache via hasattr
• resolve ConfigAttribute descriptors and annotations across the class MRO so subclasses inherit base config fields
• add a regression test proving a Config subclass can initialize with inherited fields even when attribute cache is cold

This addresses the behavior reported in #1069 where subclassed configs could ignore inherited fields and fail with default-auth errors.

Testing

• uv run --no-project --with pytest --with requests --with google-auth --with protobuf --with requests-mock --with pyfakefs -- python -m pytest tests/test_core.py -k "config_can_be_subclassed or config_subclass_inherits_attributes"
• uv run --no-project --with pytest --with pytest-mock --with requests --with google-auth --with protobuf --with requests-mock --with pyfakefs -- python -m pytest tests/test_config.py

[github-actions]

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Trigger:
go/deco-tests-run/sdk-py

Inputs:

• PR number: 1258
• Commit SHA: dc835c44ccadba8c4894b1049d4717ef71d29a7d

Checks will be approved automatically on success.