fix(bedrock): improve AWS region detection

[hkc5]

Problem

The Bedrock client was failing to detect AWS region correctly in certain scenarios:

1. When AWS_DEFAULT_REGION was set (instead of AWS_REGION), it was ignored
2. When AWS_REGION was set to an empty string, the code didn't fall back to boto3/config file
3. Users with AWS_PROFILE pointing to a profile with a region in ~/.aws/config would get us-east-1 instead of their configured region

This caused cross-region inference to fail with HTTP 403 errors when credentials were generated for a different region than the one being used.

Solution

Updated _infer_region() to:

• Check both AWS_REGION and AWS_DEFAULT_REGION environment variables
• Treat empty strings as unset (falsy), allowing fallback to boto3/config file
• Maintain backward compatibility with existing behavior (default to us-east-1 if nothing else works)

Changes

• Modified src/anthropic/lib/bedrock/_client.py - Updated _infer_region() function
• Added tests for AWS_DEFAULT_REGION support and empty string handling

Fixes #892

[hkc5]

Closing to reduce noise. Will reopen if needed.