[Bug]: /v1beta1/projects/xxx/locations/europe-west6/indexEndpoints/xxx:findNeighbors

### File Name

gemini/sample-apps/llamaindex-rag/backend/rag/index_manager.py

### What happened?

When I attempt to retrieve a query from the vector index endpoint, I receive the following error response:

`{
"error": {
"code": 404,
"message": "Could not resolve /v1beta1/projects/xxx/locations/europe-west6/indexEndpoints/xxx:findNeighbors to a method.",
"status": "UNIMPLEMENTED"
}
}`

### Relevant log output

```shell

```

### Code of Conduct

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