fix: SG-43709: ImGuiPythonBridge crash on shutdown

[markreidvfx]

Linked issues

N/A

Summarize your change.

Fix a use-after-free crash on shutdown in ImGuiPythonBridge.

The s_callbacks vector was declared as a static variable holding Python object pointers. Static destructors in C++ run after main() returns with no guaranteed order. If the vector is destroyed after Py_Finalize() has shut down the Python interpreter, Py_DECREF on the held objects crashes.

This crash does not always happen — it depends on static destructor ordering (which is undefined and varies between compilers and link order) and whether any ImGui Python callbacks were registered during the session. If the vector is empty or happens to be destroyed before Py_Finalize(), no crash occurs.

Fix by heap-allocating the callback list and adding a shutdown() call before Py_Finalize() to release Python references while the interpreter is still alive.

Describe the reason for the change.

During shutdown, C++ static destructors run after main() returns. The static s_callbacks vector held PyObject pointers with custom deleters calling Py_DECREF. If destroyed after Py_Finalize(), those deleters operate on a dead interpreter, causing a use-after-free crash.

Describe what you have tested and on which operating system.

Tested on Rocky Linux 9.

Add a list of changes, and note any that might need special attention during the review.

• ImGuiPythonBridge.cpp: Heap-allocate callback list; acquire GIL in PyObject deleter.
• ImGuiPythonBridge.h: Update callback storage type.
• PyInterface.cpp: Call ImGuiPythonBridge::shutdown() before Py_Finalize().
• DiagnosticsView.cpp: Update call to use new shutdown() API.

If possible, provide screenshots.

N/A