Fix MATLAB MEX error handling for Octave

[ProfFan]

Summary

• include mex.h with normal C++ linkage so Octave headers can declare templates
• restore the original std::cout stream buffer before mexErrMsgTxt exits the generated gateway
• update MATLAB wrapper snapshots and add focused regression coverage

Why

Octave mex.h includes C++ template declarations, which cannot appear inside an outer extern "C" block. Separately, generated gateways redirected std::cout to a stack-local mstream but restored it only after the exception handler. Because mexErrMsgTxt does not return, the error path left std::cout pointing at destroyed storage and Octave could segfault while displaying the exception.

This was reproduced while handling a failed GTSAM CustomFactor callback. With the stream restoration in place, the callback failure is reported as a normal interpreter error instead of SIGSEGV.

Testing

• uv run pytest tests/test_matlab_wrapper.py -q (17 passed)
• uv run pytest tests -q (128 passed)
• cmake .
• rebuilt the GTSAM Octave MEX and verified the failing callback exits with an error rather than signal 11