Change walk_tree to accept new mergedtreenodes for unstructured grids

[awarde96]

Change walk_tree to accept new mergedtreenodes for unstructured grids and parse correctly

Description

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• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.