[CPyCppyy] Do not decref a constructor result on Windows

[guitargeek]

CPPMethod::ExecuteFast() ends with a Windows-only block that turns a
result returned with a pending Python exception into a failure, to cover
the PyException throw not propagating there:

#ifdef _WIN32
    if (PyErr_Occurred()) {
        Py_XDECREF(result);
        result = nullptr;
    }
#endif

ExecuteFast() is shared by every CPPMethod subclass, including
CPPConstructor, whose executor is ConstructorExecutor. That one does not
return a PyObject* at all: it returns the address of the newly
constructed C++ object, cast to PyObject* for CPPConstructor::Call() to
unpack. Decref'ing that address treats arbitrary C++ object memory as a
PyObject and corrupts the heap.

Ask the method whether its executor hands back a real PyObject* before
dropping a reference. The object is leaked instead, which is acceptable
on the error path of a call that is about to be reported as failed.

🤖 Done with the help of AI

[github-actions]

Test Results

    23 files      23 suites   3d 16h 55m 26s ⏱️
 3 857 tests  3 857 ✅ 0 💤 0 ❌
78 561 runs  78 561 ✅ 0 💤 0 ❌

Results for commit 03753f1.

♻️ This comment has been updated with latest results.

[aaronj0]

Changes on the cppyy side LGTM!

[guitargeek]

Thanks for the review! I removed all non-cppyy commits from this PR, so this part can already been merged.

The part about using -W error for Python tests is anyway still blocked by deprecation warnings in the SOFIE parsers, which will go away soon:

• [tmva][sofie] Remove Keras and PyTorch parses from ROOT repository #22734

Once they are gone, I'll re-start the effort with the Python development module.