Prepare a shared test suite for validating Host–Worker contract

Create a shared test suite in a new internal repository that all Workers are required to run. The suite will include tests moved from the Host repo and newly created tests, to validate that each Worker complies with the Host–Worker contract.

Start by inventorying the tests to be moved from the Host repository and identifying the Host–Worker contract they cover. Then define the scope of the new internal repository and how every Worker will run the shared suite; done means the suite exists, includes the migrated and new tests, and is required for Worker validation.

Written by the indexing model from the issue text.