Bring 'deletion in progress' methods to Namespace::AdjournedDeletable

What does this MR do and why?

Code Changes Summary

This change consolidates deletion-related functionality by removing the standalone DeletableNamespace module and integrating its methods into the existing Namespaces::AdjournedDeletable module. The key method deletion_in_progress_or_scheduled_in_hierarchy_chain? has been moved to the AdjournedDeletable module, along with the requirement for models to implement self_deletion_in_progress?.

The Namespace class now properly overrides self_deletion_in_progress? to check the deleted_at attribute, while Project implements it to check the pending_delete? status. The ProjectNamespace class now delegates to the project's implementation.

Tests have been updated accordingly, with the DeletableNamespace tests being moved to the Namespaces::AdjournedDeletable spec file. This refactoring improves code organization by centralizing deletion-related functionality in one place while maintaining the same behavior.

References

• Related to Bring "being deleted" methods to Namespaces::Ad... (#542101 - closed) • Rémy Coutable • 18.1

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