fix: update durable execution stop test for new emulator behavior

[reedham-aws]

Why is this change necessary?

The durable execution emulator has been updated to have idempotent responses for StopDurableExecution (aws/aws-durable-execution-sdk-python-testing#195). Previously, our integration tests had been assuming that the responses were not idempotent and testing against that, causing the tests to fail.

How does it address the issue?

Update the test so that the expected error code matches what is returned in the idempotent case.\

What side effects does this change have?

Slightly less rigorous testing on the message returned from StopDurableExecution. I think this is fine because it allows us decouple from what the emulator returns for a string, in case that changes.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[reedham-aws]

Integration test run for confirmation: https://github.com/aws/aws-sam-cli/actions/runs/24146855431