Use bedrock_client region for Bedrock model ARN to prevent cross-region reranking errors

[tkawa]

Issue #, if available: N/A

Description of changes:

This pull request updates the construction of the Bedrock model ARN in builtin_tools/aws/tools/bedrock_retrieve.py (line 36) to use the region from bedrock_client instead of the previously hardcoded us-west-2.

When running in a region different from us-west-2, using a hardcoded region causes the error:
"Reranking does not support cross-region calls. Please provide a valid model ARN."

By dynamically using self.bedrock_client.meta.region_name for the ARN's region, the reranking operation will always use the same region as the client, preventing cross-region errors and improving compatibility across different AWS regions.

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[ybalbert001]

The reason I hardcode 'us-west-2' here is because there are no available rerank model access in other region.

I guess the cross-region errors is new for us. Are you sure your change is ok for every region?

[tkawa]

@ybalbert001
According to the following page, the rerank model is supported in some regions other than us-west-2.
https://docs.aws.amazon.com/bedrock/latest/userguide/rerank-supported.html

I am trying to use the bedrock knowledge base and rerank model in the ap-northeast-1 region.
This change will not make it work in all regions, but it is an improvement over the current situation, as reranking does not support cross-region calls in the first place.

[tkawa]

@warren830
What do you think about this change?