Type checking part of lint, and group build/validate statuses

[acroca]

• Run uv run mypy as part of the lint process because it doesn't require run in a matrix.
• Add a completion CI step so we can mark is as required and it'll group all checks into one requirement.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.77%. Comparing base (d612c9e) to head (d291688).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1178      +/-   ##
==========================================
- Coverage   82.84%   82.77%   -0.07%     
==========================================
  Files         123      123              
  Lines       10130    10130              
==========================================
- Hits         8392     8385       -7     
- Misses       1738     1745       +7     

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