Fix: fix the streaming issue for nova models (#580)

### 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](https://github.com/langchain-ai/langchain-aws/blob/main/libs/aws/langchain_aws/chat_models/bedrock.py#L704)).
Inside ChatBedrockConverse, the input parameter `provider` is default to
empty string rather than None
([code](https://github.com/langchain-ai/langchain-aws/blob/main/libs/aws/langchain_aws/chat_models/bedrock_converse.py#L407)),
therefore, the
[set_disable_streaming](https://github.com/langchain-ai/langchain-aws/blob/main/libs/aws/langchain_aws/chat_models/bedrock_converse.py#L582)
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

Author: 0x-fang

libs/aws/langchain_aws/chat_models/bedrock_converse.py

@@ -584,7 +584,7 @@ def set_disable_streaming(cls, values: Dict) -> Any:
 
         # Extract provider from the model_id
         # (e.g., "amazon", "anthropic", "ai21", "meta", "mistral")
-        if "provider" not in values:
+        if "provider" not in values or values["provider"] == "":
             if model_id.startswith("arn"):
                 raise ValueError(
                     "Model provider should be supplied when passing a model ARN as model_id."

libs/aws/tests/unit_tests/chat_models/test_bedrock_converse.py

@@ -1571,4 +1571,9 @@ def side_effect(service_name: str, **kwargs: Any) -> mock.Mock:
     )
 
     # The streaming should be disabled for models with no streaming support
-    assert chat_model.disable_streaming is True
+    assert chat_model.disable_streaming is True
+    
+def test_nova_provider_extraction() -> None:
+    """Test that provider is correctly extracted from Nova model ID when not provided."""
+    model = ChatBedrockConverse(client=mock.MagicMock(), model="us.amazon.nova-pro-v1:0", region_name="us-west-2")
+    assert model.provider == "amazon"