feat: 优化会话消息处理逻辑，支持在话题切换时正确保存消息并更新内存历史

src/gateway/execution.rs

@@ -110,15 +110,42 @@ impl AgentExecutionService {
 
         // 将结果消息保存到确定的话题
         if let Some(topic_id) = target_topic_id {
-            if let Err(err) = session.append_messages_to_topic(
+            if is_current_turn {
+                // 如果是最新回合，使用 append_persisted_messages 保存到数据库并更新内存历史
+                if let Err(err) = session.append_persisted_messages(
+                    request.chat_id,
+                    request.result.emitted_messages.clone(),
+                ) {
+                    tracing::error!(
+                        error = %err,
+                        chat_id = %request.chat_id,
+                        "Failed to append messages to session history"
+                    );
+                }
+            } else {
+                // 如果用户已切换话题，只保存到原始话题（不更新内存历史）
+                if let Err(err) = session.append_messages_to_topic(
+                    request.chat_id,
+                    topic_id,
+                    &request.result.emitted_messages,
+                ) {
+                    tracing::error!(
+                        error = %err,
+                        topic_id = %topic_id,
+                        "Failed to append messages to topic"
+                    );
+                }
+            }
+        } else if is_current_turn {
+            // 如果没有话题，直接更新内存历史（append_persisted_messages 会处理持久化）
+            if let Err(err) = session.append_persisted_messages(
                 request.chat_id,
-                topic_id,
+                request.result.emitted_messages.clone(),
-                &request.result.emitted_messages,
             ) {
                 tracing::error!(
                     error = %err,
-                    topic_id = %topic_id,
+                    chat_id = %request.chat_id,
-                    "Failed to append messages to topic"
+                    "Failed to append messages to session history"
                 );
             }
         }

src/providers/openai.rs

@@ -235,12 +235,13 @@ impl OpenAIProvider {
         let normalized = self.normalize_tool_arguments(arguments);
 
         if self.uses_json_tool_arguments() {
-            // Model expects JSON object format
+            // Model expects JSON object format (e.g., some code models)
             normalized
         } else {
             // Standard OpenAI format: arguments as JSON string
             // But ensure we serialize valid JSON, not null
             match normalized {
+                Value::Null => Value::String("{}".to_string()),
                 Value::String(raw) => {
                     // If the string is already valid JSON, keep it as-is
                     // Otherwise, ensure it's a proper JSON string