🐛 Fix ancillary handling in ZX checker

[burgholzer]

Description

Fixes #508

The regression tests and the corresponding CI failures demonstrate the underlying issue in the ZX checkers:

• the first C++ test, which previously segfaulted, now returns equivalent, while all other checkers agree that the two circuits are not equivalent.
• the second C++ test is a full C++ reproducer of the failing Python test. This test segfaults due to a map out of range error.
• the Python test is adapted from 🐛 Partial equivalence with ancillary qubits #508

Note the first two tests fail for separate reasons, which means there are multiple things going wrong here.

This PR adjusts the conditions under which the ZX checker declares it can handle the underlying circuits, making it more conservative than before.
Additionally, the out of range error is fixed.

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.

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[burgholzer]

@pehamTom many thanks for the changes here. I pushed a couple of commits with cleanups of unrelated changes and a small fix for an oversight in the new shortcut of the ZX Checker run method.

I only have two real questions left, which you will find inline.
Other than that, it would be nice if you could add a changelog entry for this fix.

[burgholzer]

Thanks @pehamTom for the further iteration here. I think we need one more iteration and then this should be done for good. You'll find the detailed comments inline.