* 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.
* Add unit tests for `ExecuteWorkflow` activity covering various scenarios and refactor `FlowJoin` activity to remove `[Obsolete]` attribute.
* Add integration tests for `ExecuteWorkflow` activity
- Introduce parent and child workflow definitions to test various scenarios, including input handling, output capture, correlation ID setting, and execution order.
- Add corresponding JSON workflow files for integration tests.
- Update project files to include new workflows.
* Refactor `ExecuteWorkflowTests` to consolidate repetitive code by introducing utility methods for workflow execution and instance retrieval.
* Apply suggestion from @Copilot
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
