### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

## What happened

When using Vertex AI Gemini with `stream=True` and `web_search_options={}`, LiteLLM fails mid-stream with:

```text litellm.MidStreamFallbackError: litellm.APIConnectionError: list index out of range ```

Confirmed failing models:

```text vertex_ai/gemini-3.1-flash-lite-preview vertex_ai/gemini-3.1-flash-lite vertex_ai/gemini-3-flash-preview ```

The same request pattern worked with:

```text vertex_ai/gemini-3.1-pro-preview ```

So this appears to affect Gemini Flash / Flash Lite streaming response shapes with web search, rather than all Gemini 3.x models.

## Root cause hypothesis

The failure appears to happen inside LiteLLM's streaming pipeline.

Call flow:

```text litellm.main.acompletion() -> litellm.main.completion() -> litellm.utils.get_optional_params() -> VertexGeminiConfig.map_openai_params() -> VertexGeminiConfig._map_web_search_options() -> VertexLLM.completion() -> VertexLLM.async_streaming() -> make_call() -> ModelResponseIterator.chunk_parser() -> CustomStreamWrapper.chunk_creator() -> C…