🐛 Fix Barrier handling in remove final measurements optimisation

[burgholzer]

Description

Fixes a newly introduced bug in the remove final measurements optimisation that surfaced in QCEC due to the way how Barrier gates are handled since there promotion from a NonUnitaryOperation to a StandardOperation.
Barriers are now treated similarly to measurements and a test is added to prevent future regressions.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[github-actions]

Cpp-Linter Report ✔️

No problems need attention.

Have any feedback or feature suggestions? Share it here.

[codecov]

Codecov Report

Merging #403 (6bed869) into main (819e729) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #403   +/-   ##
=====================================
  Coverage   89.3%   89.3%           
=====================================
  Files         98      98           
  Lines      11634   11627    -7     
  Branches    2084    2081    -3     
=====================================
- Hits       10392   10387    -5     
+ Misses      1242    1240    -2     

Flag	Coverage Δ
cpp	89.3% <100.0%> (+<0.1%)	⬆️
python	100.0% <ø> (ø)

Files Changed	Coverage Δ
src/CircuitOptimizer.cpp	85.2% <100.0%> (+0.1%)	⬆️