🐛 do not invalidate iterator in block collection

[burgholzer]

Description

This fixes a small bug observed as part of #803, which only surfaced on Windows in Debug mode when compiling using Clang 🤯
Part of the block collection code in the circuit optimizer would invalidate an iterator due to an update of the respective container being iterated over within the loop.

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.2%. Comparing base (bea8448) to head (89e2271).
Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #808     +/-   ##
=======================================
+ Coverage   92.0%   92.2%   +0.1%     
=======================================
  Files        125     126      +1     
  Lines      13828   13827      -1     
  Branches    2153    2153             
=======================================
+ Hits       12735   12758     +23     
+ Misses      1093    1069     -24     

Flag	Coverage Δ
cpp	92.0% <ø> (+0.1%)	⬆️
python	99.7% <ø> (ø)

Files with missing lines	Coverage Δ
src/circuit_optimizer/CircuitOptimizer.cpp	90.6% <ø> (+0.1%)	⬆️

... and 23 files with indirect coverage changes