tests: fix deprecation warnings from assert_cmd

[cakebaker]

This PR fixes deprecation warnings from assert_cmd v2.1.0 (see https://github.com/assert-rs/assert_cmd/blob/HEAD/CHANGELOG.md#210---2025-10-28)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.30%. Comparing base (b193ea0) to head (238d93e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   86.29%   86.30%   +0.01%     
==========================================
  Files          13       13              
  Lines        6222     6227       +5     
  Branches      511      511              
==========================================
+ Hits         5369     5374       +5     
  Misses        852      852              
  Partials        1        1              

Flag	Coverage Δ
macos_latest	86.24% <100.00%> (+0.01%)	⬆️
ubuntu_latest	86.36% <100.00%> (+0.01%)	⬆️
windows_latest	19.97% <0.00%> (-0.02%)	⬇️

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