fix: pydantic >= 2.0 and enforce required metadata fields and move test data to fixture

[sgomezvillamor]

Summary

Bump pydantic >= 2.0

Additionally, fixed mypy type inconsistencies and pytest collection hanging.

Changes

• Metadata model fix: Added assertions to validate required fields exist in meta_info
  • Option 1: Make fields Optional[str] ❌
  • Option 2: Keep fields as required str and fix post_init ✅ (chosen to avoid breaking changes)
• Test fixture refactoring: Moved expensive data loading from module-level to session-scoped pytest fixture
  • Prevents pytest from hanging during test collection
  • Data is loaded once when tests actually run, not during import

Testing

• mypy passes without errors
• pytest collection is now fast (no hanging)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (233a4dc) to head (2cd8792).

Additional details and impacted files

Files with missing lines	Coverage Δ
...ub-classify/src/datahub_classify/helper_classes.py	100.00% <100.00%> (ø)

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[sgomezvillamor]

not needed