feat: Improve PydanticToHFDatasets.model_to_dict()

[rmitsch]

Description

Improves PydanticToHFDatasets by

• adding support for nested model classes
• eliminating Pydantic deprecation warning
• refactoring PydanticToHFDatasets for better readability and maintainability

Related Issues

• Resolves Improve PydanticToHFDatasets.model_to_dict() #201

Changes Made

• Extend and refactor PydanticToHFDatasets

Checklist

• Tests have been extended to cover changes in functionality
• Existing and new tests succeed
• Documentation updated (if applicable)
• Related issues linked

Screenshots/Examples (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sieves/tasks/utils.py	91.52%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   93.10%   93.21%   +0.10%     
==========================================
  Files          62       62              
  Lines        3541     3554      +13     
==========================================
+ Hits         3297     3313      +16     
+ Misses        244      241       -3     

Files with missing lines	Coverage Δ
sieves/tasks/utils.py	93.75% <91.52%> (+4.59%)	⬆️

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