Fix LightCone sparse observable ordering

[Christopher Rivera (chrisriv10)]

Summary

Fixes #16866.

LightCone reverses the positional association between sparse observable bit_terms and indices.

Sparse observable terms associate positionally as bit_terms[i] -> indices[i], while PauliGate uses Qiskit's dense Pauli ordering, where the rightmost character acts on local qubit 0. Passing bit_terms directly to PauliGate therefore reverses asymmetric multi-qubit observables.

This can cause LightCone to perform commutation checks against the wrong observable and incorrectly remove gates, changing expectation values.

Details

• Reverse bit_terms when constructing the initial PauliGate, preserving the positional association with indices.
• Add a regression test for an asymmetric "XZ" observable on qubits [0, 1].
• Update two existing test expectations that depended on the reversed ordering.
• Add a release note for the bug fix.

Testing

• python -m pytest test/python/transpiler/test_light_cone.py -v — 22 passed.
• ruff check qiskit/transpiler/passes/optimization/light_cone.py test/python/transpiler/test_light_cone.py — passed.
• black --check qiskit/transpiler/passes/optimization/light_cone.py test/python/transpiler/test_light_cone.py — passed.

AI/LLM disclosure

• No part of this submission is LLM generated.
• Some written text was generated by: ChatGPT (OpenAI), Claude (Anthropic)
• Some submitted code was generated by: AI coding agents using OpenAI and Anthropic models

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[Jake Lishman (jakelishman)]

Thanks for the interest. I've left a discussion comment that we need to resolve before we can decide what the correct fix is: #16866 (comment).