Qiskit / qiskit-fermions / 33624751028

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feat: add `OperatorTrait.is_hermitian` (#304)

* feat: add is_hermitian to the OperatorTrait protocol

Every operator type already provided this check, but it was reachable only
through the concrete classes: neither the Rust trait nor the Python protocol
declared it, so generic code over `T: OperatorTrait` (or a parameter annotated
`OperatorTrait`, such as `jordan_wigner`'s) could not call it. Several sessions
have now run into that gap.

It is a required trait method rather than one with a default body because the
check reduces `self - self.adjoint()` to a normal form, and `normal_ordered` is
not part of the trait: it takes different arguments on every operator type, so
there is no single call a default body could make. Hoisting a normal-form hook
onto the trait to enable one was rejected, because the contract of such a hook
differs per type in exactly the way that decides this method's strength, and it
would be a new core API whose only caller is this method. A default body also
cannot reach `__sub__`, which lives on `OperatorMacro`.

That per-type strength difference is the reason the trait declaration and the
Python protocol document only the weakest guarantee, with the two operator types
that have something specific to say documenting it on their own implementation:
`EdgeVertexOperator`'s normal form contracts every reducible pair, so its result
is exact, while `TransferVertexOperator`'s leaves shared-mode transfer products
alone, so a `False` result there can be a false negative.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* refactor: drop the inherent num_groups shadowing the trait default

`OperatorTrait::num_groups` has a default body, but all four operator types also
declared an inherent method of the same name. Rust resolves `op.num_groups()` to
the inherent one, so the trait default was dead code for every concrete type and
only ever ran through a generic bound. The two implementations agreed, but
nothing enforced that: had the... (continued)

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