test(workflow_ops): unit tests for module helpers

[pratyush618]

Summary

Adds 8 inline unit tests to crates/taskito-python/src/py_queue/workflow_ops/mod.rs covering the three pure helpers and the cascade_skip_pending_nodes orchestrator that the rest of the workflow_ops submodules build on:

• build_metadata_json — JSON output is correctly escaped for special characters and Unicode (regression coverage for the bug whose fix is documented inline).
• parse_step_metadata — happy-path round-trip + invalid-JSON error path.
• status_to_py — canonical state string mapping.
• cascade_skip_pending_nodes — pending/ready nodes get skipped, terminal/running nodes left alone, attached jobs get cancelled, and an empty node slice is a no-op.

The PyO3 entry-point methods (mark_workflow_node_result, check_fan_out_completion, expand_fan_out, gate ops, etc.) are not covered here. They take Python<'_> and call py.allow_threads and self.storage — testing them as Rust unit tests would require constructing a full PyQueue plus a Python interpreter. The underlying state transitions they wrap (finalize_fan_out_parent, update_workflow_run_state, update_workflow_node_status) are already tested in the taskito-workflows crate's contract suite. The remaining gap is the PyO3 glue, which is exercised by the Python workflow test suite end-to-end.

Addresses recommendation 7 from the 2026-05-08 audit (.claude/reports/audit-2026-05-08.md).

Test plan

• cargo test --workspace --features workflows — 8 new tests pass alongside the existing suite.
• cargo clippy --workspace --features workflows — clean.
• cargo fmt --all --check — clean.