Support JET@0.12 in tests

[devmotion]

Updates JET version used for tests on Julia >= 1.12 to JET@0.12, and makes use of the new JET.JET_AVAILABLE gate in JET@0.12 (JET's new policy is to be Pkg-compatible with newer unreleased Julia version but only functionally compatible with those for which JET.JET_AVAILABLE = true).

Closes #518. Closes #535. Closes #519.

Note: CI failure on pre is unrelated and fixed by #536

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.28%. Comparing base (e477922) to head (780d892).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #537      +/-   ##
==========================================
- Coverage   94.34%   94.28%   -0.07%     
==========================================
  Files          14       14              
  Lines        3026     3026              
==========================================
- Hits         2855     2853       -2     
- Misses        171      173       +2     

Flag	Coverage Δ
unittests	94.28% <ø> (-0.07%)	⬇️

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