fix: update size policy for auto clim button in LUT widget

[tlambert03]

This uses a fixed size policy on the Autoscale button rather than a maximum width

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.79%. Comparing base (709f779) to head (78e9da6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   86.79%   86.79%           
=======================================
  Files          50       50           
  Lines        5725     5725           
=======================================
  Hits         4969     4969           
  Misses        756      756           

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