Preview (dry-run) a commitment purchase without creating it

Prices a prospective commitment purchase and returns its derived economics WITHOUT persisting anything — a dry-run for the commitment creation form savings preview. Takes the same CreateCommitmentInput payload as the create operation (commitment type, term, payment option, committed hourly rate in ten-thousandths of the currency unit per hour, and the covered services) and returns the effective discount fraction from the commitment discount table together with the derived on-demand list rate, committed rate, covered quantity per hour, projected upfront and projected monthly savings. The figures are computed against the REAL rate card exactly as the create derivation would, so the client no longer has to mirror the backend discount table to render an accurate preview. Validation: identical to create — type/term/payment must be valid enum members; hourly must be a positive integer (ten-thousandths of the currency unit per hour); services must be a non-empty array of known service ids (unknown id → 422 VALIDATION_FAILED); a covered family with no rated SKU in the scope's region → 422 VALIDATION_FAILED. Caller needs a billing-capable role (owner|admin|billing), else 403. Async & state: Synchronous; NO state change — no commitment row is created and nothing is charged. Errors: 401 unauthenticated; 403 insufficient role; 404 unknown scope; 422 field validation failure; 500 internal failure. Idempotency/Concurrency: Safe to repeat; no idempotency key required — the call never mutates.