Fix Bedrock region detection to respect aws_profile and AWS_DEFAULT_REGION

[Sarmkadan]

Summary

Fixes #892

The Bedrock client's _infer_region() function had two issues causing incorrect region detection:

• aws_profile not forwarded: When a user passed aws_profile="my-profile" to AnthropicBedrock(), the profile parameter was not passed through to _infer_region(), so boto3.Session() was created without the profile name. This meant the profile-specific region from ~/.aws/config was ignored, and the client would fall back to us-east-1 instead of using the profile's configured region. (The AWS_PROFILE env var still worked because boto3 reads it internally, but the explicit constructor parameter did not.)

• AWS_DEFAULT_REGION not checked: The function only checked the AWS_REGION env var but not AWS_DEFAULT_REGION, which is also supported by boto3 and the AWS CLI.

Changes

• _infer_region() now accepts an aws_profile parameter and passes it to boto3.Session(profile_name=...)
• AWS_DEFAULT_REGION is checked as a fallback when AWS_REGION is unset
• self.aws_profile is assigned before _infer_region() is called so the profile is available
• Removed redundant or "us-east-1" fallbacks in _prepare_request (both sync and async) since _infer_region() already guarantees a non-None return

Resolution order (updated)

1. AWS_REGION environment variable
2. AWS_DEFAULT_REGION environment variable
3. Region from boto3.Session (respects AWS_PROFILE env var, ~/.aws/config, and the explicit aws_profile constructor parameter)
4. Hard-coded us-east-1 (legacy fallback)

Test plan

• Added test_region_infer_from_aws_default_region - verifies AWS_DEFAULT_REGION is respected
• Added test_aws_region_takes_precedence_over_aws_default_region - verifies AWS_REGION wins over AWS_DEFAULT_REGION
• Added test_region_infer_from_explicit_aws_profile_param - verifies the aws_profile constructor param is used for region inference (sync client)
• Added test_region_infer_from_explicit_aws_profile_param_async - same for async client
• All 16 existing + new tests pass