feat: (WIP) improve openrouter tool parsing and openai tool encoding

backend/src/tool.py

@@ -127,10 +127,10 @@ class ToolCall:
         is_error: Whether the result represents an error.
     """
 
-    def __init__(self, tool_name, arguments=None, id=None):
+    def __init__(self, tool_name, arguments={}, id=None):
         self.id = id or ""
         self.tool_name = tool_name
-        self.arguments = arguments or {}
+        self.arguments = arguments
 
         self.result = None