fix: add error handling and logging to model generator scripts

[harz05]

Fixes #15 (SOFIE equivalent of root-project/root#21190)

Changes

All 6 model generator scripts in src/SOFIE_core/test/ had no error handling. This adds:

• try/except around torch.onnx.export() calls so failures are caught and logged
• validation that the output .onnx file exists and is non-empty after export
• sys.exit(1) on failure so CI can detect it
• Fixed unclosed file handles by using with open() instead of bare open()
• added logging for progress and error messages

Files changed:

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

All changes are consistent across the 6 files.