HSSM base class

[cpaniaguam]

This pull request refactors the handling of missing data and deadline logic in the HSSM codebase by moving this functionality from the DataValidatorMixin into a new, dedicated MissingDataMixin. The HSSM class now inherits from both mixins, improving code organization and separation of concerns. Additionally, the associated tests have been updated to reflect these changes, and some redundant or outdated tests have been removed.

Key changes:

Refactoring & Code Organization:

• Introduced a new MissingDataMixin in src/hssm/missing_data_mixin.py, which encapsulates all logic related to handling missing data and deadlines, including data validation, error handling, and processing routines. This makes the codebase more modular and maintainable.
• Updated the HSSM class in src/hssm/hssm.py to inherit from both DataValidatorMixin and the new MissingDataMixin, ensuring that missing data and deadline handling is now managed by the new mixin. [1] [2]
• Removed the now-obsolete _handle_missing_data_and_deadline method from DataValidatorMixin in src/hssm/data_validator.py as its functionality has been moved to the new mixin.

Testing Updates:

• Removed tests for _handle_missing_data_and_deadline from tests/test_data_validator.py, as this logic is now encapsulated in the new mixin and tested elsewhere.
• Cleaned up test code in tests/test_hssm.py by removing unused variables and simplifying calls to HSSM and its methods, reflecting updated code patterns and improving test clarity. [1] [2] [3] [4]