Add error handling and logging to Python model generator scripts

[harz05]

Related Issue: This is the ML4EP/SOFIE equivalent of root-project/root#21190.

In root-project/root#21190, @dpiparo explicitly asked for this to be reopened here first

Problem

The Python model generator scripts in src/SOFIE_core/test/ currently have no error handling:

• LinearModelGenerator.py
• Conv1dModelGenerator.py
• Conv2dModelGenerator.py
• Conv3dModelGenerator.py
• ConvTrans2dModelGenerator.py
• RecurrentModelGenerator.py

If a model export fails silently, there is no way to tell which step failed, no meaningful exit code and no validation that the .onnx file was actually created.

Proposed fix

• Wrap model export calls in try/except so failures are caught and logged
• Validate the .onnx file exists and is non-empty after each export
• Exit with sys.exit(1) on failure so CI can detect it