seperate concerns of PR #15022 to create a new branch with just the f…

[aaryav3]

• Separate concerns of PR Add SparseObservable.to_matrix, migrate from SparsePauliOp #15022 Add SparseObservable.to_matrix, migrate from SparsePauliOp #15022 [to create a new branch with just the fast-pauli path. SparseObservable undergoing this method, with projectors besides paulis exits with an exception (untested, this gap will be filled by Add SparseObservable.to_matrix, migrate from SparsePauliOp #15022)

See discussion here with @alexanderivrii

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code: sonnet-4.7 (to cherrypick and re-add comments)

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