bedrock: Fix unchecked error when stream channel is closed

[lucaronca]

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As you can see in the aws sdk, when an error is set then stream channel is closed:

• https://github.com/aws/aws-sdk-go-v2/blob/main/service/bedrockruntime/eventstream.go#L84
• https://github.com/aws/aws-sdk-go-v2/blob/main/service/bedrockruntime/eventstream.go#L85
• https://github.com/aws/aws-sdk-go-v2/blob/main/service/bedrockruntime/eventstream.go#L71

The stream channel was read, but once closed, the error was not checked.
You can reproduce a bug triggering a streaming completion response and cancelling the context. The context.Canceled error is not returned.
I'm adding it just for anthropic provider because is the only one that supports streaming at present.

[tmc]

should cover in test but LGTM