[MEDIUM] create_hierarchy is not one atomic Undo operation and property writes bypass Undo

[Daliys]

Problem

create_hierarchy creates a tree, adds components, and applies serialized properties without wrapping the whole client-facing operation in one Undo group. Property application also bypasses Undo.RecordObject.

Evidence

• Editor/MCPServerMethods.Hierarchy.cs:47-56 recursively creates objects and mutates them after Undo.RegisterCreatedObjectUndo.
• Editor/MCPServerMethods.Hierarchy.cs:61-68 adds components through Undo.AddComponent per component.
• Editor/MCPServerMethods.Hierarchy.cs:71-107 applies component properties via SerializedObject.ApplyModifiedProperties() without Undo.RecordObject.
• There is no Undo.SetCurrentGroupName / Undo.CollapseUndoOperations around the whole hierarchy creation.

Impact

A single create_hierarchy RPC can produce multiple Undo entries and non-undo-tracked property changes, making the operation non-atomic from a Unity user perspective.

Suggested fix

Wrap the full operation in a named Undo group, record component objects before property mutation, and collapse the group before returning.

Source report

Imported from audit report items:

• unity-antipattern (6) / Editor/MCPServerMethods.Hierarchy.cs:47
• unity-antipattern (6) / Editor/MCPServerMethods.Hierarchy.cs:71

Grouped because both describe the same create_hierarchy Undo contract gap.