Validate model and network block dispatch

[wshlavacek]

Summary

• resolve model and network block dispatch through explicit supported-name maps instead of raw getattr lookup
• preserve supported aliases like reaction_rules while raising a clear ValueError for unsupported block names
• add focused upstream-safe tests for both supported and unsupported dispatch paths

Testing

• uv run pytest tests/test_block_dispatch_validation.py -q
• uv run pytest tests/test_bionetgen.py::test_bionetgen_model tests/test_bng_parsing.py::test_network_parse -q
• uvx black --check bionetgen/modelapi/model.py bionetgen/network/network.py tests/test_block_dispatch_validation.py