Updated app example

[jpolvto]

Added an updated version of this app, which now includes LangGraph. The Bing Search API has been sunset, so it's not included anymore.

Interestingly, this adds similar functionality to Open WebUI or AnythingLLM, but in under 1000 lines of code.

[jpolvto]

One thing I still needed to include here was the StreamHandler, the LangchainCallbackHandler still does not handle agents out of the box it seems.

`# Callback handler for handling streaming responses from the language model
class StreamHandler(BaseCallbackHandler):
"""
A callback handler for handling streaming responses from the language model.

Attributes:
    msg (cl.Message): The message object used for streaming the response.

Methods:
    on_llm_new_token: Called when a new token is received from the language model.
    on_llm_end: Called when the streaming response from the language model ends.
"""

def __init__(self):
    self.msg = cl.Message(content="")

async def on_llm_new_token(self, token: str, **kwargs):
    await self.msg.stream_token(token)

async def on_llm_end(self, response: str, **kwargs):
    await self.msg.send()
    self.msg = cl.Message(content="")`