Minor fixes to FTSP

[pehamTom]

Description

Fixed edgede case in heuristic state prep for codes with even distances.
Furthermore changed calculation of logical error rate to be calculated per qubit

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 82.7%. Comparing base (bc00666) to head (6ac26e6).
Report is 125 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #243   +/-   ##
=====================================
  Coverage   82.7%   82.7%           
=====================================
  Files         29      29           
  Lines       3170    3170           
  Branches     190     190           
=====================================
  Hits        2623    2623           
  Misses       547     547           

Flag	Coverage Δ		*Carryforward flag
cpp	80.2% <ø> (ø)		Carriedforward from 40d3427
python	83.7% <100.0%> (ø)

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
src/mqt/qecc/ft_stateprep/simulation.py	85.5% <100.0%> (ø)
src/mqt/qecc/ft_stateprep/state_prep.py	75.7% <100.0%> (ø)