Fix: Fixes empty input in the langchain stream calls

[noble-varghese]

Description

• Fixes the input in the stream calls by adding the inputs in chain streaming and astream calling

Issue: langfuse/langfuse#1589

[hassiebp]

@noble-varghese Do I understand correctly from your changes that the underlying issue was that vor .achat chains there was no input on the on_chain_start callback, but only delivered in the on_chain_end callback?

[noble-varghese]

@hassiebp Yes, in the case of the streaming data the input passed on the on_chain_start method from langchain contains a dummy input and not the actual input passed by the user. The complete input is only delivered at the end of the chain on the on_chain_end method.
You can check the code here from langchain where the input is being passed as a dummy input for the streaming data.