Fix optional union types in generated Python signatures

[huxiaoyi-ovo]

Summary

Fix generated Python signatures for optional pybind11 arguments that use PEP 604 union syntax such as T | None.

Open3D's docstring parser did not allow | in argument types, so signatures emitted by pybind11 3 could be truncated before stub generation. For compute_fpfh_feature, this made the optional indices argument appear required in the generated type stub.

This change:

• allows | in parsed argument type expressions;
• adds a regression test for the optional compute_fpfh_feature(..., indices=None) signature.

Fixes #7583.

Validation

The branch was validated against Open3D's CI before submission:

• full Python suite passed: 8435 passed, 21 skipped, 1 xfailed;
• the new FPFH signature regression test passed;
• Windows CPU/CUDA/XPU builds passed;
• macOS builds and wheel test passed;
• Ubuntu, SYCL, OpenBLAS, and ARM64 checks passed.

One fork CI job reported failure only in the final wheel artifact upload step after build/tests succeeded: the workflow searches for open3d-[0-9]*.whl, while the produced wheel is named open3d_cpu-*.whl. No code or test step failed.

[update-docs]

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