### Check for existing issues

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

### What happened?

# AttributeError: `LiteLLMCompletionStreamingIterator` object has no attribute `completed_response` on mid-stream provider error (Anthropic via Responses API)

## What happened

When the Anthropic provider returns a **mid-stream error** (e.g. HTTP 529 Overloaded / a 5xx) while streaming a **Responses API** completion, litellm's fallback/cleanup path raises:

``` AttributeError: 'LiteLLMCompletionStreamingIterator' object has no attribute 'completed_response' ```

This `AttributeError` **masks the real error** (the underlying `MidStreamFallbackError` / provider overload) and escapes the normal streaming-error contract. Because the surfaced exception is now a bare `AttributeError` rather than a stream/incomplete error, any downstream retry or model-fallback logic keyed on litellm's streaming exceptions never runs, and the call fails hard instead of falling back.

## Root cause (as far as we can tell)

On a mid-stream provider error, litellm raises `MidStreamFallbackError` and then attempts to recover partial token usage by reading `source_iterato…