* 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.
* Restore EF Core persistence layer
Originally, we had moved EF Core to the extensions repo, but in practice this turned out to be rather impractical when debugging elsa-core, given that we would lose all of the state (workflow definitions, instances, etc.) across application restarts.
* Integrate PostgreSQL support into EF Core persistence and enhance BlobStorage workflows provider setup.
