Fix: fix the streaming issue for nova models

[0x-fang]

Description

This PR fixes the bug that cause streaming is broken for Amazon Nova model. The response was returned as a whole rather than streaming back chunks to customers, even though .stream() is used.

Root Cause and Fix

First of all, when using Amazon Nova model, langchain-aws will use ChatBedrockConverse even though we create ChatBedrock (code). Inside ChatBedrockConverse, the input parameter provider is default to empty string rather than None (code), therefore, the set_disable_streaming function will use provider="" to disable the streaming, even though the model can support streaming. To fix it, this PR updates set_disable_streaming function to extract provider from model Id correctly when provider is empty string as default value, so it can setup the streaming flag correctly.

Alternatively, user can set the provider parameter explicitly when initializing the ChatBedrock as below, which can make streaming work for Nova models without any code change.

llm = ChatBedrock(
    provider="amazon",
    model="us.amazon.nova-pro-v1:0",
)

for chunk in llm.stream(prompt):
    print(chunk)

Issue

#544

[michaelnchin]

Thank you @0x-fang !