Qiskit / qiskit-fermions / 30527531774

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feat(operators): add has_groups() to the Python API (#233)

Checking whether an operator tracks groups had to go through the `groups`
attribute, i.e. `op.groups is None`. That getter clones the entire group
index vector out of Rust just to have it discarded again, so the cheapest
possible question -- "is this `Option` populated?" -- was paying a copy
proportional to the number of terms.

`has_groups()` answers it without touching the indices. The C API already
had `qf_{ferm,maj}_op_has_groups`; this brings the Python API up to par
and covers all four operator types, not just the two the C API exposes.

Also corrects the two C API docstrings, which claimed has_groups reports a
`groups` attribute "that is not empty". The implementation has always been
`is_some()`, and an empty-but-present `groups` is reachable in practice
(`from_fcidump` seeds `Some(vec![])`), so `true` with zero indices is a
real state -- now documented as such on both the Rust and Python side
rather than contradicted.

Refs #229

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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