feat(provider/google-vertex): Add support for the imageSize provider option (#10651)

## Background

New sampleImageSize option for Google Vertex provider with Imagen:
https://docs.cloud.google.com/vertex-ai/generative-ai/docs/model-reference/imagen-api#sample_image_size_rest

## Summary

Added the option in the vertex providerOptions and updated related test

Author: alex-deneuvillers

.changeset/healthy-donuts-stare.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/google-vertex': patch
+---
+
+feat(provider/google-vertex): Add support for the imageSize provider option

packages/google-vertex/src/google-vertex-image-model.test.ts

@@ -1,7 +1,7 @@
 import { createTestServer } from '@ai-sdk/test-server/with-vitest';
+import { describe, expect, it, vi } from 'vitest';
 import { GoogleVertexImageModel } from './google-vertex-image-model';
 import { createVertex } from './google-vertex-provider';
-import { describe, it, expect, vi } from 'vitest';
 
 vi.mock('./version', () => ({
   VERSION: '0.0.0-test',
@@ -308,6 +308,7 @@ describe('GoogleVertexImageModel', () => {
             addWatermark: false,
             negativePrompt: 'negative prompt',
             personGeneration: 'allow_all',
+            sampleImageSize: '2K',
             foo: 'bar',
           },
         },
@@ -321,6 +322,7 @@ describe('GoogleVertexImageModel', () => {
           negativePrompt: 'negative prompt',
           personGeneration: 'allow_all',
           aspectRatio: '16:9',
+          sampleImageSize: '2K',
         },
       });
     });

packages/google-vertex/src/google-vertex-image-model.ts

@@ -145,6 +145,7 @@ const vertexImageProviderOptionsSchema = z.object({
     .nullish(),
   addWatermark: z.boolean().nullish(),
   storageUri: z.string().nullish(),
+  sampleImageSize: z.enum(['1K', '2K']).nullish(),
 });
 export type GoogleVertexImageProviderOptions = z.infer<
   typeof vertexImageProviderOptionsSchema