fix(code/app): Fix bug in test app and example app where proposal parts were not always processed in order of sequence number

[romac]

Closes: #XXX

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PR author checklist

For all contributors

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

For external contributors

• Maintainers of Malachite are allowed to push changes to the branch

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.35%. Comparing base (0968a34) to head (a4458ea).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #851   +/-   ##
=======================================
  Coverage   82.35%   82.35%           
=======================================
  Files         186      186           
  Lines       15951    15953    +2     
=======================================
+ Hits        13136    13138    +2     
  Misses       2815     2815           

Flag	Coverage Δ
integration	82.19% <ø> (+<0.01%)	⬆️
mbt	8.46% <ø> (ø)

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Components	Coverage Δ
core	∅ <ø> (∅)
engine	∅ <ø> (∅)
app	∅ <ø> (∅)
starknet	∅ <ø> (∅)