fix(core/loader): Handle non-mapping YAML input gracefully

[sentry]

Replaced NurseSchedulingData(**data) with NurseSchedulingData.model_validate(data) in core/nurse_scheduling/loader.py.

This change addresses a TypeError that occurred when a user uploaded a YAML file whose root element was not a mapping (e.g., a scalar string). The previous implementation would attempt to unpack a string using **data, leading to a TypeError.

By using Pydantic's model_validate(data), the system now correctly handles non-mapping inputs by raising a ValidationError, providing a more robust and user-friendly error message instead of a low-level Python TypeError.

Fixes NURSE-SCHEDULING-5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.57%. Comparing base (06926ee) to head (9ea032e).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #43   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files          77       77           
  Lines        5220     5220           
  Branches      704      704           
=======================================
  Hits         4780     4780           
  Misses        293      293           
  Partials      147      147           

Flag	Coverage Δ
core	94.05% <100.00%> (ø)
core-macos	94.05% <100.00%> (ø)
core-windows	94.05% <100.00%> (ø)
web-frontend	88.75% <ø> (ø)
web-frontend-macos	88.75% <ø> (ø)
web-frontend-windows	88.75% <ø> (ø)

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Files with missing lines	Coverage Δ
core/nurse_scheduling/loader.py	100.00% <100.00%> (ø)

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