Create DI, Designer, and scope lifecycle tests and run them in CI

## Objective

Add runtime unit/integration tests and the appropriate Designer tests for assignment, activation, nested scopes, disposal, exceptions, design-time behavior, and backward compatibility. Run locally and in CI, clearly separating runtime and closed-source Designer validation ownership.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

Start by mapping the runtime DI, Designer, and scope-lifecycle test areas and identifying the existing test and CI entry points. Define coverage for assignment, activation, nested scopes, disposal, exceptions, design-time behavior, and backward compatibility, with runtime and closed-source Designer validation ownership documented; done means the tests run locally and in CI.

Written by the indexing model from the issue text.