Function start_streaming_with_fallback 

pub async fn start_streaming_with_fallback(
    config: &StreamingConfig,
    prompt: &str,
    tool_schemas: Option<ProviderToolSchemas>,
    tx: Sender<StreamChunk>,
    cancel_token: CancellationToken,
) -> Result<(), AppError>

Start streaming with fallback to secondary provider on failure Implements jittered exponential backoff with rate-limit-aware delay selection before falling back.