fix(gateway): guard against missing finishReason in doGenerate response

[s-zx]

Summary

When using the Vercel AI gateway with Google image-generation models, the deserialized response body may omit the finishReason field entirely. The existing doGenerate implementation spreads responseBody directly into its return value, so when finishReason is absent the downstream code in generate-text.ts crashes with:

TypeError: Cannot read properties of undefined (reading 'unified')

(line 984: currentModelResponse.finishReason.unified)

Root cause

GatewayLanguageModel.doGenerate performs a simple object spread of the raw gateway response:

return {
  ...responseBody,
  request: { body: args },
  response: { headers: responseHeaders, body: rawResponse },
  warnings,
};

Google image-generation model responses served through the gateway omit finishReason, leaving the field undefined after the spread.

Fix

Add a nullish-coalescing fallback in packages/gateway/src/gateway-language-model.ts so that a missing finishReason defaults to { unified: 'other', raw: undefined }:

return {
  ...responseBody,
  finishReason: responseBody.finishReason ?? {
    unified: 'other',
    raw: undefined,
  },
  request: { body: args },
  response: { headers: responseHeaders, body: rawResponse },
  warnings,
};

This matches the behaviour expected by generate-text.ts and every other provider in the SDK, while remaining a no-op for gateway responses that already supply a finishReason.

Fixes #13626