Mishandling of model with single configured region

I don't think the `else` block in the below section of Chainlit app.py is behaving as expected for users wanting to configure a single region:

https://github.com/aws-samples/foundational-llm-chat/blob/c502b461360833fa2d03f873b3a3680c4abd318d/chainlit_image/foundational-llm-chat_app/app.py#L291-L297

If the expected config for a single region is an array e.g. `"region": ["us-west-2"]`, then we should be passing `model_info["region"][0]` because [boto3 Config](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html) expects a single string not an array.

If the expected config would be `"region": "us-west-2"`, then the check will mis-behave because `len("us-west-2")` is 9. I think you'd want an explicit handler e.g. `isinstance(model_info["region"], str)`

Start with chainlit_image/foundational-llm-chat_app/app.py at lines 291-297 and compare the region handling with boto3 Config's single-string requirement. Confirm whether the configuration accepts a one-element array or a string, then verify that both the selected region and multi-region behavior are passed correctly. Done means single-region configuration no longer produces an invalid region value.

Written by the indexing model from the issue text.