dont throw if mock response file isn't found

Author: dlarocque

packages/vertexai/test-utils/mock-response.ts

@@ -49,9 +49,6 @@ export function getMockResponseStreaming(
   filename: string,
   chunkLength: number = 20
 ): Partial<Response> {
-  if (!(filename in mocksLookup)) {
-    throw Error(`Mock response file not found: '${filename}'`);
-  }
   const fullText = mocksLookup[filename];
 
   return {