test(code): Add some unit tests to the core-consensus crate

[romac]

Closes: #431

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PR author checklist

• Reference GitHub issue
• Ensure PR title follows the conventional commits spec

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 11 lines in your changes missing coverage. Please review.

Project coverage is 79.41%. Comparing base (9768451) to head (f4bc22f).
Report is 56 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
code/crates/engine/src/consensus.rs	9.09%	10 Missing ⚠️
code/crates/app/src/spawn.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #673      +/-   ##
==========================================
+ Coverage   79.35%   79.41%   +0.06%     
==========================================
  Files         104      104              
  Lines        9020     9023       +3     
==========================================
+ Hits         7157     7165       +8     
+ Misses       1863     1858       -5     

Flag	Coverage Δ
integration	79.12% <8.33%> (+0.06%)	⬆️
mbt	16.59% <ø> (-3.50%)	⬇️

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