Qiskit / qiskit-fermions / 39767609940

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fix(circuit): reject non-positive Suzuki-Trotter orders (#393)

The order check only ruled out odd values, so 0 and negative even orders
passed the constructor. The recursion in the schedule builder only stops
at orders 1 and 2, so any such order descended forever and surfaced as a
RecursionError at synthesis time, far from the offending argument. An
operator with a single factor short-circuits before the recursion, so
whether it failed also depended on the operator.

Validate in the constructor instead: the order must be 1 or a positive
even integer.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

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