test: fix expected SubType for wait_for_callback in tests

[valerena]

Expect WaitForCallback instead of RunInChildContext subtype.

Which issue(s) does this change fix?

Durable Functions changed the subtype for wait_for_callback() in their last version: aws/aws-durable-execution-sdk-python#476

Why is this change necessary?

Tests were failing

How does it address the issue?

Update the tests to check for the new values coming from the durable-functions SDK.

What side effects does this change have?

Nothing. Just tests.

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.