Fix incorrect placement of z-errors

[pehamTom]

Description

Due to a missing check in the propagation of errors, some Z-errors were propagated incorrectly

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 (172a381) to head (f9d9baa).
Report is 112 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #272   +/-   ##
=====================================
  Coverage   82.6%   82.7%           
=====================================
  Files         46      46           
  Lines       3920    3978   +58     
  Branches     353     353           
=====================================
+ Hits        3241    3292   +51     
- Misses       679     686    +7     

Flag	Coverage Δ		*Carryforward flag
cpp	83.3% <ø> (ø)		Carriedforward from 172a381
python	82.3% <100.0%> (+0.1%)	⬆️

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

Files with missing lines	Coverage Δ
src/mqt/qecc/ft_stateprep/state_prep.py	72.1% <100.0%> (+0.4%)	⬆️

... and 7 files with indirect coverage changes

[burgholzer]

LGTM

Really just a comment because I stumbled upon it: we shouldn't be relying on private properties of Qiskit (Bit._index).
This is subject to change and access to the index (via the public Bit.index) is actually deprecated since way before Qiskit 1.0.

[pehamTom]

I am currently refactoring code anyway since I am working on something new that overlaps a lot with the state prep functionality. Gonna get rid of those private accesses then.