test: Collect coverage from Rust code

[borchero]

Motivation

With plenty of logic moving to Rust, it would make sense to ensure good coverage there as well.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.45%. Comparing base (91961d5) to head (7eab2d1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #203      +/-   ##
===========================================
- Coverage   100.00%   99.45%   -0.55%     
===========================================
  Files           53       61       +8     
  Lines         2996     3288     +292     
===========================================
+ Hits          2996     3270     +274     
- Misses           0       18      +18     

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