Convert tests to TestItems framework

[andreasnoack]

Summary

• Migrate all tests from @testset/include() to @testitem blocks (TestItems.jl) with TestItemRunner.jl for execution
• Shared test utilities (_default_rtol) wrapped in a @testmodule Utils setup block
• Each test file is now a self-contained @testitem with its own using statements

Test plan

• Pkg.test() passes (1,166,082 tests pass, 2 expected broken)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.39%. Comparing base (37d3c86) to head (0d5a095).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #209   +/-   ##
=======================================
  Coverage   72.39%   72.39%           
=======================================
  Files          22       22           
  Lines        1007     1007           
=======================================
  Hits          729      729           
  Misses        278      278           

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