Unify networkparser, structs, and rulemod error handling

[wshlavacek]

Summary

• Replace debug and print-on-invalid-input paths in networkparser, structs, and rulemod with parse errors.
• Add focused regression coverage for malformed species lines, line_label fallback behavior, and invalid rule/rulemod contracts.

Testing

• uv run python -m pytest -q tests/test_networkparser_structs_errors.py tests/test_bng_parsing.py tests/test_get_rule_mod.py
• uvx black --check bionetgen/network/networkparser.py bionetgen/modelapi/structs.py bionetgen/network/structs.py bionetgen/modelapi/rulemod.py tests/test_networkparser_structs_errors.py