fix: Improve bedrock provider extraction from model id

[lucaronca]

Currently, the bedrock implementation assumed that the model ID would start with the provider name, which meant it can't handle cases like passing the ARN as the model ID or where model ID uses a different format. For instance, AWS customer support recommends using a custom-formatted model ID to work around a bedrock bug (see: AWS Repost). I have now updated the code to extract the provider using a more flexible approach that supports these custom formats.

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