fix: deepseek streaming 400 error (remove stream_options.include_usage)

src/providers/deepseek.rs

@@ -113,7 +113,8 @@ impl super::Provider for DeepSeekProvider {
     ) -> Result<BoxStream<'static, Result<ProviderStreamChunk, AppError>>, AppError> {
         // DeepSeek doesn't support images in streaming, use text-only
         let messages_json = helpers::messages_to_openai_json_text_only(&request.messages).await?;
-        let body = helpers::build_openai_body(&request, messages_json, true);
+        let mut body = helpers::build_openai_body(&request, messages_json, true);
+        body.as_object_mut().expect("body is object").remove("stream_options");
 
         let es = reqwest_eventsource::EventSource::new(
             self.client