Relax validator set id constraints

[vgeddes]

Allow the candidate nextValidatorSet ID to be 1 or more than the current nextValidatorSet id.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.60%. Comparing base (e7fa333) to head (5162a26).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1729      +/-   ##
==========================================
+ Coverage   75.20%   76.60%   +1.40%     
==========================================
  Files          24       24              
  Lines         996      979      -17     
  Branches      186      186              
==========================================
+ Hits          749      750       +1     
+ Misses        223      205      -18     
  Partials       24       24              

Flag	Coverage Δ
solidity	76.60% <100.00%> (+1.40%)	⬆️

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[yrong]

So we're allowing non-continuous session ID switches — for example, (current,next) moving from (3,4) to (4,8). In that case,do we also want to allow a subsequent commitment whose validator set ID falls in between (e.g., 5)?

If that’s the case, the changes in this PR might not be sufficient, as this could also break other checks, such as:

snowbridge/contracts/src/BeefyClient.sol

Line 293 in e7fa333

} else if (commitment.validatorSetID == nextValidatorSet.id) {

This also reminds me of a previous PR:
#1137

[yrong]

+1