[Bug]: discoveryengine.SearchRequest can't find results

### File Name

https://github.com/GoogleCloudPlatform/generative-ai/blob/main/search/vertexai-search-options/vertexai_search_options.ipynb

### What happened?

I followed the instruction and run the code.
But it always return - > summary: "No results could be found. Try rephrasing the search query."
while I was using search_sample function.

It is weird, because it can run with curl instruction with [Google cloud / AI Applications / Apps / {app name} / Integration / API ]

Is there any possible induce it dose't work with python code?
Btw, I always use the same search query. It can run with curl but not python code.

### Relevant log output

```shell
search_response = search_sample(
PROJECT_ID, VERTEX_AI_SEARCH_LOCATION, VERTEX_AI_SEARCH_APP_ID, SEARCH_QUERY
)
search_response.summary.summary_with_metadata

-> summary: "No results could be found. Try rephrasing the search query."
citation_metadata {
}
```

### Code of Conduct

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