fix(tools): make image_understand image_url optional and remove unused operation variable

Change image_url from required to optional in both operation_tool.py and
tool_service.py for image_understand operation, avoiding parameter validation
conflict with uploaded_files priority logic.
Remove unused operation variable from OpenClawTool.execute().

api/app/services/tool_service.py

@@ -730,8 +730,8 @@ class ToolService:
                 {
                     "name": "image_url",
                     "type": "string",
-                    "description": "必须提供，要分析的图片 URL 或 base64 data URI",
-                    "required": True
+                    "description": "要分析的图片 URL 或 base64 data URI",
+                    "required": False
                 }
             ]
         else: