Update calls of `arun` to `ainvoke` for langchain chat models

[tanmaygupta9]

Using the arun methods to call a chat model -- which is what we currently do -- gives the following warning: LangChainDeprecationWarning: The function arun was deprecated in LangChain 0.1.0 and will be removed in 0.2.0. Use ainvoke instead.

arun will be removed in langchain==0.2.0 (currently at 0.1.14) so we should update this sooner rather than later

[tanmaygupta9]

• don't arun an AgentExecutor, instead update calls to ainvoke.