[iqm] Don't reject user-named measurement registers

[mitchdz]

PR #4818 added a numerical sort of the IQM measurement_keys to reorder result bitstrings into physical-qubit order, guarded by a check that every key ends in a digit. That assumption only holds for the auto-generated r00000, r00001, ... register names. A kernel that names its measurement register explicitly (e.g. auto result = mz(q)) produces a key such as result, which tripped the guard and threw "Malformed measurement key received: result", aborting the job.

The unit tests exercised only digit-suffixed keys via the mock server, so this passed CI at merge time but broke the nightly IQM integration test (targettests/iqm/callable_kernel_arg.cpp) every night since.

Non-numeric keys carry no positional information, so there is nothing to sort by. Skip the reordering in that case and leave the bitstrings in the order the measurements were emitted -- the behavior prior to #4818 -- rather than rejecting the response. Numeric keys keep the #4818 reordering.

Adds a regression test covering a non-numeric measurement key.

[schweitzpgi]

Seems like a good change. Should this look for the "rXXXX" format to be a small step safer? Also, a malicious user could easily use "r00000" as a name to mess that up. What fun.

[sacpis]

Adding @iqm-bhoffmann for viz.

[github-actions]

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[iqm-bhoffmann]

@sacpis thank you for adding me to the conversation.
I was not aware of this way to generate names (keys) for measurements. So my change assumed keys consisting of a fixed alphabetic prefix followed by a sortable numeric part. This helps to sort the results from our backend to resolve #4621 .

The proposed change will suppress the exception for the presented testcase but will most likely make #4621 re-appear with kernels which name the measurements. If I can confirm this I will re-open #4621 to do a more sophisticated solution than just sorting the keys. In the meantime the change can be merged but could have the described side effect.

[iqm-bhoffmann]

Opened PR #5074 with my proposal to fix the problem. This solves also my concern from my last comment that multiple named measurements would still fail.