Rebuild an existing plan with fresh pricing, without losing slippage protection

Use the refresh object from a prior SwapPlan response (or GET/derive buildId from buildId field) instead of calling POST /swap again with the same quoteId — this endpoint enforces two things POST /swap alone doesn't: the slippage ceiling never loosens across refreshes (effectiveSlippageCap), and if the underlying route's final destination address changed (not just repriced — a genuinely different router), it fails closed with 409 route_changed instead of silently swapping you into a different route. When the original provider disappears from the fan-out entirely, the 409 response embeds the current best route ready to use in error.details.alternative, so you don't need a third round trip.