[donkeyProgramming]

PR Classification

Major refactor and modernization of the pack file browser and context menu system for extensibility, testability, and maintainability.

PR Summary

This pull request unifies and modernizes the pack file tree and context menu architecture, introduces new abstractions for dialogs and file system access, and overhauls the test suite for improved coverage and maintainability.

• TreeNode.cs: Removes TreeNodeSource, consolidates all tree logic into TreeNode, and adds helpers for manipulation, path navigation, and state management.
• DependencyInjectionContainer.cs: Updates DI to register new context menu registry, composer, and dialog/file system abstractions.
• StandardDialogs.cs: Refactors to use new dialog abstraction and improves file/folder dialog handling.
• Test suite: Rewrites and expands tests for the new tree model and context menu commands, including new helpers and focused fixtures.
• Removes obsolete code and improves documentation throughout the codebase.