Migrate from API Key to Auth Token for sample tests

The LocalStack API key (`LOCALSTACK_API_KEY`) has been sunset and will no longer work for authentication. To ensure that the sample tests in this repository continue to execute successfully, migrate all usages of `LOCALSTACK_API_KEY` to the new `LOCALSTACK_AUTH_TOKEN` environment variable.

**Tasks:**
- Identify all places where `LOCALSTACK_API_KEY` is referenced and replace with `LOCALSTACK_AUTH_TOKEN` (mainly in `makefile.yml`)
- Update documentation and code comments where necessary
- Validate that sample tests pass with the new auth token

**Impact:**

Without this migration, sample tests will fail due to authentication errors since the API key mechanism is deprecated.