## Summary

When the Gemini AI Studio backend (`generativelanguage.googleapis.com`) returns a 5xx response, `litellm.acompletion` raises `litellm.llms.custom_httpx.http_handler.MaskedHTTPStatusError` instead of the typed `litellm.exceptions.ServiceUnavailableError`. Downstream code that wraps `litellm.acompletion` and catches `litellm.exceptions.*` cannot distinguish an upstream service-unavailable failure from any other unclassified error without also depending on the internal `litellm.llms.custom_httpx.http_handler` wrapper class.

## Environment

- `litellm==1.85.1` - Python 3.13 - Provider: `gemini/gemini-3-flash-preview` - Upstream endpoint observed: `https://generativelanguage.googleapis.com/v1alpha/models/gemini-3-flash-preview:generateContent`

## Reproduction

1. Configure LiteLLM with a Gemini model alias (`gemini/gemini-3-flash-preview`). 2. Issue an `await litellm.acompletion(model="gemini/gemini-3-flash-preview", messages=[...])` at a time when the Google AI Studio backend returns 503. 3. Observe the exception type raised by `acompletion`.

## Actual behavior

``` Traceback (most recent call last): File "/app/.venv/lib/python3.13/site-packages/litellm/llms/custom_htt…