Unify modelapi export and XML error handling

[wshlavacek]

Summary

• Convert modelapi export and XML parsing paths from print/False returns to explicit error contracts.
• Add focused upstream tests for export failure cases and XML parser contract failures.
• Touches bngparser.py; rebases trivially on Strip stray unquoted backslashes in action normalization #89 and Collapse repeated commas in action normalization #90 while those remain open.

Testing

• uv run python -m pytest -q tests/test_modelapi_export_errors.py tests/test_xmlparsers_errors.py tests/test_get_rule_mod.py tests/test_bng_parsing.py
• uvx black --check bionetgen/modelapi/bngfile.py bionetgen/modelapi/bngparser.py bionetgen/modelapi/model.py bionetgen/modelapi/xmlparsers.py tests/test_modelapi_export_errors.py tests/test_xmlparsers_errors.py