fix(aws): Correctly handle serialization for model_kwargs in ChatBedrockConverse

[michaelnchin]

Fixes: #713

Updated the build_extra model args validator in ChatBedrockConverse to correctly handle direct model_kwargs user input. Previously, this would interfere with the call to langchain-core's _build_model_kwargs, which would throw a Circular reference detected error during JSON serialization.

Also added a user warning to switch to the correct additional_model_request_fields accepted by both ChatBedrockConverse, per the underlying API.

[3coins]

Left a minor comment about the log messages, looks good otherwise.

[3coins]

Looks good! Thanks.