Management API: Refactor element tree controllers to use start node filter service

[lauraneto]

Summary

• Refactors UserStartNodeFolderTreeControllerBase to delegate start node filtering to IUserStartNodeTreeFilterService, matching the pattern established for document and media trees in Management API: Reduce user start node tree filtering code duplication #22486
• Introduces IElementStartNodeTreeFilterService and ElementStartNodeTreeFilterService to handle element-specific start node resolution (both Element and ElementContainer object types)
• Adds a virtual TreeObjectTypes property to UserStartNodeTreeFilterService so subclasses can supply multiple object types without changing the base class contract
• Updates all element tree controllers (Root, Children, Ancestors, Siblings) to use the new service via DI

This is the v18 counterpart to #22486 (v17), covering the element tree controllers that were introduced in v18. The original v18 PR was #22016 (closed).

Testing

Open the backoffice and navigate to the element tree. Verify that root, children, ancestors, and sibling tree operations behave the same as before for both admin users (root access, no filtering) and restricted users (with element start nodes configured). Confirm that folder nodes and element nodes both appear correctly in the tree.

[github-actions]

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