Unify block deletion and invalid-action errors

[wshlavacek]

Summary

• make missing block-item deletion raise the underlying key/index error instead of printing and continuing
• let invalid action types fail through the centralized action validation path instead of printing and returning
• add focused upstream-safe contract tests for the deletion and invalid-action paths

Testing

• uv run pytest tests/test_block_error_contracts.py -q
• uv run pytest tests/test_bionetgen.py::test_action_loading -q
• uvx black --check bionetgen/modelapi/blocks.py bionetgen/network/blocks.py tests/test_block_error_contracts.py