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.

[marko1olo]

CI note: the two non-CLA failures are the same stochastic VPE test failure, not from the optimal Givens mutation path changed in this PR.

Failing test in both Python code coverage tests and Unit tests (ubuntu-24.04, 1.4.1):

src/openfermion/circuits/vpe_circuits_test.py::test_single_timestep
AssertionError: assert 0.0107... < 0.01

The test comment says the standard deviation is about 0.005 and the bound should pass about 95% of the time, so this appears to be the expected occasional sampling tail just over the 1e-2 threshold.