Avoid mutating optimal Givens input matrices

[marko1olo]

Fixes #761.

What changed

• Copy the caller-provided unitary before the optimal Givens decomposition uses it as an in-place workspace.
• Add regression coverage proving repeated decompositions with the same matrix leave the input unchanged and produce the same circuit.

Verification

• .venv\Scripts\python.exe -m pytest -p no:cacheprovider src\openfermion\circuits\primitives\optimal_givens_decomposition_test.py -q
• .venv\Scripts\python.exe -m black --check --workers 1 src\openfermion\circuits\primitives\optimal_givens_decomposition.py src\openfermion\circuits\primitives\optimal_givens_decomposition_test.py
• .venv\Scripts\python.exe -B -c "import ast, pathlib; [ast.parse(pathlib.Path(p).read_text(encoding='utf-8'), filename=p) for p in ['src/openfermion/circuits/primitives/optimal_givens_decomposition.py','src/openfermion/circuits/primitives/optimal_givens_decomposition_test.py']]"
• git diff --check

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[gemini-code-assist]

Code Review

This pull request ensures that the optimal_givens_decomposition function does not mutate its input unitary matrix by creating a copy of it. A corresponding unit test has been added to verify that the input matrix remains unmodified and that consecutive function calls produce identical circuits. There are no review comments, so I have no feedback to provide.