docs: add standalone test helpers usage reference

[N-i-xx]

Summary

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item
• Contract behavior and invariants are described clearly
• Docs updated if contract interfaces or workflows changed
• Scope stays limited to one contract concern
  closes Add docs for using the contract test helpers in isolation #350

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