fix: preserve DeepSeek V4 thinking history

rust/crates/api/tests/openai_compat_integration.rs

@@ -159,6 +159,11 @@ async fn send_message_preserves_deepseek_reasoning_content_before_text() {
             },
         ]
     );
+
+    let captured = state.lock().await;
+    let request = captured.first().expect("server should capture request");
+    let body: serde_json::Value = serde_json::from_str(&request.body).expect("json body");
+    assert_eq!(body["thinking"], json!({"type": "enabled"}));
 }
 
 #[tokio::test]