refactor(models): remove unused _arun method from LangChain tool (#69)

The _arun async method was just wrapping the sync _run method without
providing any actual async benefits. LangChain's BaseTool provides a
default implementation that raises NotImplementedError, which is
sufficient for our use case since we don't support true async execution.

Author: ryoppippi

stackone_ai/models.py

@@ -437,9 +437,6 @@ class StackOneLangChainTool(BaseTool):
             def _run(self, **kwargs: Any) -> Any:
                 return parent_tool.execute(kwargs)
 
-            async def _arun(self, **kwargs: Any) -> Any:
-                return self._run(**kwargs)
-
         return StackOneLangChainTool()
 
     def set_account_id(self, account_id: str | None) -> None: