Make MATLAB handle deletion idempotent

[ProfFan]

Summary

• delete generated MATLAB native handles only while their pointer remains collector-owned
• make repeated explicit or automatic destruction a no-op instead of a double free
• add regression coverage and refresh generated C++ snapshots

Validation

• python3 -m unittest tests.test_matlab_wrapper (14 passed)
• python3 -m pytest -q -o addopts='' tests (126 passed on the rebased branch)
• regenerated and rebuilt the GTSAM bindings with Octave
• verified both delete(v); clear v and delete(v); delete(v); clear v exit successfully
• confirmed generated MATLAB sources are byte-for-byte identical before and after rebasing onto codex/pybind-direct-function-pointers