Add initial pydantic model

[kaitj]

An initial pydantic model that tried to mimic the descriptor schema found in niwrap. Tested it locally against some descriptors and those have passed. Still needs a little work to integrate with the core.

Some additional notes:

• Wrapping the validation with try-except ends up with more helpful error messages (at least when developing).
• Might be missing some validation logic
• Briefly discussed writing two base model classes and using Union for oneOf cases (from the descriptor).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 190 lines in your changes missing coverage. Please review.

Project coverage is 63.70%. Comparing base (a2b8e53) to head (f434749).

Files with missing lines	Patch %	Lines
src/styx/frontend/schema/properties/io.py	0.00%	85 Missing ⚠️
src/styx/frontend/schema/descriptors_schema.py	0.00%	33 Missing ⚠️
src/styx/frontend/schema/properties/containers.py	0.00%	18 Missing ⚠️
src/styx/frontend/schema/properties/tests.py	0.00%	13 Missing ⚠️
src/styx/frontend/schema/properties/groups.py	0.00%	11 Missing ⚠️
src/styx/frontend/schema/properties/__init__.py	0.00%	8 Missing ⚠️
src/styx/frontend/schema/properties/resources.py	0.00%	8 Missing ⚠️
src/styx/frontend/schema/properties/environment.py	0.00%	6 Missing ⚠️
src/styx/frontend/schema/__init__.py	0.00%	4 Missing ⚠️
src/styx/frontend/schema/properties/errors.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   71.37%   63.70%   -7.67%     
==========================================
  Files          20       30      +10     
  Lines        1579     1769     +190     
==========================================
  Hits         1127     1127              
- Misses        452      642     +190     

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