* Enhance null-safety annotations across modules and refactor for improved consistency:
- Added `null!` annotations to enforce non-nullability expectations.
- Updated workflows, tests, and runtime services to handle default null values reliably.
- Removed obsolete and unused APIs, simplifying interfaces and improving maintainability.
- Refactored methods and properties for clarity, thread-safety, and consistency.
- Adjusted test configurations for code coverage tracking and integration improvements.
* Refactor activity iteration in container serialization tests to simplify type casting.
Introduce a new integration test to verify workflow serialization functionality, ensuring that newly created workflow definitions can be serialized and deserialized correctly. Adjust default nullability for certain fields in `WorkflowDefinition` to improve consistency and prevent null reference issues. Minor argument update in `New` method of `WorkflowDefinitionPublisher`.
