feat: 优化工具参数序列化逻辑，确保字符串格式的有效性

src/providers/openai.rs

@@ -128,12 +128,24 @@ impl OpenAIProvider {
         let normalized = self.normalize_tool_arguments(arguments);
 
         if self.uses_json_tool_arguments() {
+            // Model expects JSON object format
             normalized
         } else {
+            // Standard OpenAI format: arguments as JSON string
+            // But ensure we serialize valid JSON, not null
             match normalized {
-                Value::String(raw) => Value::String(raw),
+                Value::String(raw) => {
+                    // If the string is already valid JSON, keep it as-is
+                    // Otherwise, ensure it's a proper JSON string
+                    if serde_json::from_str::<Value>(raw).is_ok() {
+                        Value::String(raw)
+                    } else {
+                        // Invalid JSON string - wrap it as a proper JSON string
+                        serde_json::to_string(&raw).unwrap_or_else(|_| "null".to_string())
+                    }
+                }
                 value => Value::String(
-                    serde_json::to_string(&value).unwrap_or_else(|_| "null".to_string()),
+                    serde_json::to_string(&value).unwrap_or_else(|_| "{}".to_string()),
                 ),
             }
         }