feat(gateway): add GatewayFailedDependencyError (424)

[R-Taneja]

What

Adds a general-purpose GatewayFailedDependencyError and maps the Gateway failed_dependency error type (HTTP 424 Failed Dependency) to it.

Not provider-specific — any Gateway response where the request couldn't be fulfilled because a dependency wasn't available on the serving credentials/provider.

import { GatewayFailedDependencyError } from '@ai-sdk/gateway';

catch (error) {
  if (GatewayFailedDependencyError.isInstance(error)) {
    // error.statusCode === 424, error.type === 'failed_dependency'
  }
}

Why

Today a failed_dependency type hits the default case of createGatewayErrorFromResponse and becomes GatewayInternalServerError (type: 'internal_server_error'). The HTTP status (424) and isRetryable (false) are already preserved correctly — only the type is mislabeled as an internal server error. This adds the matching case so the type is accurate.

• isRetryable === false (424 is outside the 408/409/429/≥500 retry set), so it won't be auto-retried by retryWithExponentialBackoff.
• Follows the existing one-class-per-type pattern; only adds a case + class + exports, default untouched.

First consumer: the Gateway-side explicit-cache fallback refusal (vercel/ai-gateway#2177).

Tests

• New case in create-gateway-error.test.ts (type, message, 424, isRetryable: false).
• Full gateway suite green (429 node + 429 edge), type-check + lint clean, changeset included.