gateway IAM role trust policy uses profile default region instead of --region argument

list the sample in which the bug is present.

01-tutorials/02-AgentCore-gateway/04-integration/02-runtime-gateway-mcp-toolkit/agentcore_toolkit/utils.py

Bug Description

The create_agentcore_gateway_role() function in utils.py derives the AWS region
from the default boto3 session (Session().region_name) instead of using the
--region argument passed to the toolkit. This causes the IAM role trust policy's
ArnLike condition to contain the wrong region.

For example, when deploying with --region us-east-1 but the AWS profile defaults
to eu-west-1, the trust policy is created with:

"aws:SourceArn": "arn:aws:bedrock-agentcore:eu-west-1::*"

instead of:

"aws:SourceArn": "arn:aws:bedrock-agentcore:us-east-1::*"

This prevents the AgentCore gateway service from assuming the role, causing all
tools/call requests to fail with:

"InternalServerException - Failed to obtain execution role credentials"

Note: tools/list still works because it only requires the gateway to route the
request, not assume the execution role.

The same pattern exists in create_agentcore_role() and
create_agentcore_gateway_role_s3_smithy() — all three functions should accept
a region parameter rather than deriving it from the default session.

Fix: Add a region parameter to these functions and pass self.region from the
toolkit's main.py call site.