Async video flow supported; doStart retries idempotent

Patch Changes

• 89080c8: feat (provider/gateway): support the async video operation flow (doStart/doStatus) on the v4 video model

• 89080c8: fix (ai/gateway): make retried doStart calls idempotent

  generateVideo retries doStart, which creates a billable generation, so a retry after a lost response could start a second one. It now mints one idempotency token per logical start — outside the retry closure — and forwards it as an idempotency-key header, so a provider that deduplicates (the Vercel AI Gateway does) sees the same key on every attempt. GatewayVideoModel simply forwards the caller's headers rather than inferring retry identity from an options object, which would collide across unrelated calls.