Fix multi-qubit noise broadcast in Stim MSM mode

[bmhowe23]

Note: it's debatable whether we should pursue merging this, as this bug only seems to affect MSM mode, which could go away now that the new DEM mode is being used, but I am posting anyway.

Related: NVIDIA/cudaqx#606

A noise channel attached to a multi-qubit gate is broadcast independently across each group of num_targets qubits in the sampling path (via Stim's multi-target semantics) and the DEM path. The MSM path, which injects errors into x_table/z_table by hand, only iterated num_targets and indexed qubits[0], so for a single-qubit-tagged channel applied to >1 qubit (e.g. the QEC library's two_qubit_depolarization / two_qubit_bitflip) every qubit past the first was silently dropped from the MSM and any MSM-derived DEM, while the sampled syndromes still contained that noise.

Make the MSM branch mirror the broadcast: enumerate each group of num_targets qubits, giving each group its own error id, and update the msm_size pass's mechanism count to match. Throw if the qubit count is not a positive multiple of num_targets.

Add Stim MSM regression tests for the two-qubit depolarization and bit-flip broadcast cases.

[github-actions]

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