fix(bedrock): use boto3 Session region from aws_profile instead of hardcoded us-east-1

[giulio-leone]

Fixes #892

Problem

When AWS_REGION is unset, AnthropicBedrock falls back to a hardcoded us-east-1 region instead of reading the region from the user's AWS profile. This means users who configure a non-default region via aws_profile (e.g., eu-west-1, ap-southeast-1) are silently routed to us-east-1, causing cross-region inference failures or unexpected latency. The issue is particularly confusing because the user has correctly configured their AWS profile but the SDK ignores it.

Root Cause

The _infer_region() helper did not accept or use the aws_profile parameter. It checked only the AWS_REGION / AWS_DEFAULT_REGION environment variables and, if neither was set, returned us-east-1 unconditionally — even when a valid profile with a configured region was available.

Fix

• _infer_region() now accepts an optional aws_profile parameter and creates a boto3.Session(profile_name=aws_profile) to read the configured region.
• Both sync (AnthropicBedrock) and async (AsyncAnthropicBedrock) constructors forward their aws_profile argument to _infer_region().
• Falls back to us-east-1 only when no profile is provided and no environment variable is set, preserving backward compatibility.

Testing

Two new tests verify:

• Profile-based region detection returns the correct region from boto3.Session
• Graceful fallback to us-east-1 when no profile or environment region is available

[giulio-leone]

Friendly ping — CI is green, tests pass, rebased on latest. Ready for review whenever convenient. Happy to address any feedback. 🙏