Replace mypy with ty

[tomvothecoder]

Description

Replace MyPy with ty in development and pre-commit tooling, then resolve ty diagnostics and add focused regression coverage for the required behavior changes.

• No associated issue.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

❌ Patch coverage is 98.48485% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.95%. Comparing base (3bef023) to head (81cc551).

Files with missing lines	Patch %	Lines
xcdat/regridder/xgcm.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #844      +/-   ##
===========================================
- Coverage   100.00%   99.95%   -0.05%     
===========================================
  Files           18       18              
  Lines         2012     2040      +28     
===========================================
+ Hits          2012     2039      +27     
- Misses           0        1       +1     

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