Support AWS_REGION env var, cluster context fallback, and centralize boto3 client creation

[FarhanTejani]

What's changing and why?

Two related changes:

1. Fix AWS_REGION env var being ignored (closes #357)

The CLI ignores the AWS_REGION environment variable. The AWS CLI respects both AWS_REGION and AWS_DEFAULT_REGION, but boto3 only checks AWS_DEFAULT_REGION. This is confusing for users who expect consistent behavior.

This change introduces a _resolve_region function with the following fallback order:

1. Explicit --region flag
2. AWS_REGION env var
3. AWS_DEFAULT_REGION / ~/.aws/config (standard boto3 chain)
4. Region from current cluster context (extracted from the EKS ARN set by set-cluster-context)

Step 4 means that after running hyp set-cluster-context, subsequent commands automatically use that cluster's region without needing --region.

2. Centralize boto3 client creation

The codebase had 13 direct boto3.client() calls scattered across files, bypassing the existing create_boto3_client utility. This meant any cross-cutting concern (region resolution, user-agent, retry config) had to be patched in N places instead of one.

All direct boto3.client() calls are now routed through create_boto3_client, and unused boto3 imports are removed. The only remaining direct boto3 usage is in utils.py (where create_boto3_client is defined) and cluster.py (which uses boto3.Session for credential validation).

Before/After UX

Before:

$ AWS_REGION=us-west-2 hyp list-cluster
# ERROR - Please ensure you have configured the AWS default region

After:

$ AWS_REGION=us-west-2 hyp list-cluster
# Works, lists clusters in us-west-2

After set-cluster-context:

$ hyp set-cluster-context --cluster-name my-cluster --region us-west-2
$ hyp list-cluster
# Works without --region, uses region from cluster context

How was this change tested?

• Unit tests pass
• Verified AWS_REGION=us-west-2 hyp list-cluster returns cluster data
• Verified AWS_DEFAULT_REGION still works
• Verified --region flag still takes precedence
• Verified inference commands using get_current_region() are unaffected

Are unit tests added?

Yes, added tests for _resolve_region:

• Explicit region takes precedence over env vars
• AWS_REGION env var is used when no explicit region
• Falls back to boto3 default when no env var
• Falls back to cluster context when boto3 has no region
• Returns None when nothing is configured

Are integration tests added?

N/A

Reviewer Guidelines

‼️ Merge Requirements: PRs with failing integration tests cannot be merged without justification.

One of the following must be true:

• All automated PR checks pass
• Failed tests include local run results/screenshots proving they work
• Changes are documentation-only