feat: use useq-schema GridFromPolygon

[fdrgsp]

Add GridFromPolygon from useq-schema.

This PR replace the local GridFromPolygon logic and uses the GridFromPolygon form uses-schema.

It also updates MDAWidget to be able to show (but not really edit) a GridFromPolygon.

Screen.Recording.2025-08-13.at.10.32.45.AM.mov

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.82%. Comparing base (9dd18a0) to head (ee8149b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pymmcore_widgets/useq_widgets/_grid.py	94.36%	8 Missing ⚠️
src/pymmcore_widgets/control/_rois/roi_model.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
+ Coverage   87.45%   87.82%   +0.37%     
==========================================
  Files         105      105              
  Lines       11476    11555      +79     
==========================================
+ Hits        10036    10148     +112     
+ Misses       1440     1407      -33     

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