### What happened?

When using the LiteLLM proxy's `/v1/messages` endpoint (which goes through the router via `anthropic_messages` route type), mid-stream errors from Anthropic (e.g. `overloaded_error`, `internal_server_error` SSE events) are not caught by the router's fallback system.

### Root Cause

The `_ageneric_api_call_with_fallbacks_helper` method in `router.py` returns the streaming `AsyncIterator` directly without wrapping it in a fallback-aware wrapper. Compare with the `acompletion` path which wraps streaming responses in `FallbackStreamWrapper` via `_acompletion_streaming_iterator` (router.py:2162-2167).

For the generic path at router.py:3855, the response is simply `return response` with no error detection or fallback wrapping.

### Expected Behavior

When Anthropic returns HTTP 200 and starts streaming, then sends an SSE error event (`event: error` with `overloaded_error` or `internal_server_error`), the router should detect this mid-stream error and attempt to fall back to a configured fallback model (e.g. Bedrock).

### Actual Behavior

The SSE error event passes through silently to the client. The request is logged as `status=success` in the spend logs. No fallba…