Spot Shell (v0.20.2)

Update AWS tests for current behavior and optional credentials

Adjust AWS package tests to match current launch template override and
allowed-AZ filtering behavior, removing coverage for deleted helpers.
Add a shared AWS config loader that skips integration-style tests when
credentials or region are unavailable, and update affected image, key,
and VPC tests to use it so `go test github.com/mikeb26/spotsh/aws`
passes without code changes.

Co-authored-by: Octium Agent <agent@octium.dev>