Collapse repeated commas in action normalization

[wshlavacek]

Summary

• extend _normalize_action_text() to collapse repeated commas that appear outside quoted strings
• preserve repeated commas inside quoted strings
• add focused regression coverage in tests/test_bng_parsing.py

Why

BNG2.pl tolerates malformed action text like n_steps=>20,,print_functions=>1, but the pyparsing action parser does not.

Testing

• uv run python -m pytest -q tests/test_bng_parsing.py
• uvx black --check bionetgen/modelapi/bngparser.py tests/test_bng_parsing.py