Use region from boto3 session when possible

[zastrowm]

Description

Only use a default region if the session doesn't provide one or if the AWS_REGION environment variable is not set. Previously we were using us-west-2 even if the session could load it from the environment (either variables or profiles). No we only specify a region if the session doesn't provide one

Fixes #238

Related Issues

#238

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

I tested creating an agent:

• With a profile region set (boto3 default behavior)
• With an environment variable AWS_REGION specified (our custom behavior)
• With an environment variable AWS_DEFAULT_REGION specified (boto3 default behavior)
• Explicitly passing a region
• Using the default (us-west-2)

All cases used the region as expected

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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