tests/_internal/test_frozen_dataclass_sealable.py(style): Fix formatting and docstring style

why: Tests must follow the project's style guidelines for readability and maintainability.

what:
- Fixed docstring formatting to match reStructuredText format
- Added blank lines between summaries and descriptions
- Split long lines to comply with 88-character limit
- Improved test documentation for clarity
- Restructured tests to better demonstrate edge cases
- Ensured consistent formatting throughout

refs: Ensures tests follow project coding standards

Author: tony