fix(deepseek): preserve reasoning_content in history for R1

DeepSeek R1 (deepseek-reasoner) requires that assistant messages in the history
containing tool_calls must also include their reasoning_content field.

src/providers/helpers.rs

@@ -60,6 +60,11 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
 
         let mut msg = serde_json::json!({ "role": m.role });
 
+        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
+        if let Some(reasoning) = &m.reasoning_content {
+            msg["reasoning_content"] = serde_json::json!(reasoning);
+        }
+
         // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {
@@ -133,6 +138,11 @@ pub async fn messages_to_openai_json_text_only(
 
         let mut msg = serde_json::json!({ "role": m.role });
 
+        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
+        if let Some(reasoning) = &m.reasoning_content {
+            msg["reasoning_content"] = serde_json::json!(reasoning);
+        }
+
         // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {