fix: preserve multi-image prompt batches

scripts/test-invariants.ts

@@ -90,10 +90,16 @@ const ATTACHMENT_COMPANION = '../packages/attachment/attachment-local/src/invari
 class TestAttachmentStore extends AttachmentStore {
   readonly imageLimits: ImageAttachmentLimits = {
     maxImageBytes: 1,
+    maxImagesPerMessage: 1,
+    maxMessageImageBytes: 1,
     maxImagePixels: 1,
     mediaTypes: ['image/png'],
   }
 
+  validateImage(_input: SaveImageAttachment): void {
+    throw new Error('test invariant attachment store does not validate images')
+  }
+
   saveImage(_input: SaveImageAttachment): Promise<ImageAttachmentRef> {
     return Promise.reject(new Error('test invariant attachment store does not save images'))
   }