feat: support AWS_REGION environment variable for region configuration

[tbrand]

Description

Under the current specification, setting the AWS_REGION environment variable and running agentcore launch results in an error because the region is not properly configured.

The reason AWS_REGION is not being read is that boto3 irregularly reads the AWS_DEFAULT_REGION environment variable instead. The status of environment variables for other CLIs and SDKs is summarized in the following link:

https://stackoverflow.com/questions/59961939/what-is-the-difference-between-aws-default-region-and-aws-region-system-variable

I believe we should align with the AWS standard specification rather than boto3's specific behavior. Therefore, I've made changes so that when AWS_REGION is set, agentcore launch will execute in that specified region.

This change is categorized as a breaking change because if AWS_REGION is unintentionally set, agentcore launch might execute in an unintended region. However, I believe such cases would be rare.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring

Testing

• Unit tests pass locally
• [-] Integration tests pass (if applicable)
• Test coverage remains above 80%
• Manual testing completed

Checklist

• My code follows the project's style guidelines (ruff/pre-commit)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Security Checklist

• No hardcoded secrets or credentials
• No new security warnings from bandit
• Dependencies are from trusted sources
• No sensitive data logged

Breaking Changes

List any breaking changes and migration instructions:

If unintended deployment to a different region occurs due to AWS_REGION, you will need to either remove or override the AWS_REGION environment variable. AWS_DEFAULT_REGION remains available for use, but AWS_REGION takes higher precedence.

Additional Notes

N/A