Swift: Finalize Node resolution architecture

Let's leave this for now, but in the shared design from @andrewkolos , we're leaning towards putting the node resolution stuff in a separate class, mostly for separation of concerns. The model classes mostly represent data, and the node resolution follows references to build the tree representation.

_Originally posted by @jacobsimionato in https://github.com/a2ui-project/a2ui/pull/2052#discussion_r3635778432_

Review the linked PR discussion #2052 to understand the proposed architecture for node resolution. Look at the existing model classes and identify where node resolution logic currently resides. The goal is to extract this logic into a separate class to improve separation of concerns.

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