Validate a cart without creating anything. Use this to render a
"review your order" screen showing taxes and any platform-charged fees
the customer will pay, plus warn the customer about out‑of‑stock
items.

Caveats

• The endpoint always returns 200 even when there are validation
  errors. Always inspect data.errors (top level) and
  data.lines[].error (per line) before continuing to POST /orders.
• Numeric totals can drift by ±0.01 between two calls if the merchant
  changes prices in between. Always re-validate immediately before
  placing.
• Disabled / removed products on the location are reported as
  Item not available and won't appear in data.lines. Their absence
  will appear in data.errors.
• Validate is read-only — nothing is persisted on Ewity's side.