fix(code/core-consensus): Only cancel propose timeout on a step change

[romac]

See: #850

@ancazamfir: When a Proposal(h, r, v, vr) is received without a PolkaPrevious, consensus cancels the propose timeout therefore never reaching the Prevote step and starting vote sync. (#850 (comment))

When receiving a proposal ⁠Proposal(h, r, v, vr) with ⁠vr > -1 without a polka for vr, we must:

1. Stay in the Propose step
2. Keep the Propose timeout active to either:
   • Collect more votes to form a polka, prevote ⁠v and move to the Prevote step
   • Let the Propose timeout expire, prevote ⁠nil and move to the Prevote step

Previously, we were incorrectly cancelling the Propose timeout in all cases, including that one, violating (2).

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• Reference GitHub issue
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[ancazamfir]

👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.39%. Comparing base (b8ff51e) to head (099d611).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #862      +/-   ##
==========================================
+ Coverage   82.10%   82.39%   +0.29%     
==========================================
  Files         187      187              
  Lines       16012    16022      +10     
==========================================
+ Hits        13146    13200      +54     
+ Misses       2866     2822      -44     

Flag	Coverage Δ
integration	82.23% <ø> (+0.29%)	⬆️
mbt	8.41% <ø> (-0.01%)	⬇️

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