Fix unhydrated node parent tracking

[CraigMacomber]

Description

Fix unhydrated node parent tracking by centralizing the logic to update parents, and applying it consistently after every edit.

This does make small edits in large sequences do O(N) parent updates, but they were already O(N) from modifying the large array, and need to do O(N) parent updates in the insertion is near the front anyway.

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[Copilot]

Pull Request Overview

This PR fixes unhydrated node parent tracking by centralizing the logic for updating parent pointers after every edit.

• Centralizes parent updates in the edit() method of UnhydratedFlexTreeField
• Updates tests to validate correct key and parent tracking for both insertions and removals

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/dds/tree/src/test/simple-tree/api/treeNodeApi.spec.ts	Updates tests with new hydration descriptions and behaviors for key and parent tracking
packages/dds/tree/src/simple-tree/core/unhydratedFlexTree.ts	Refactors parent updates by clearing and resetting adoptBy in the edit() method, and removes redundant adoptBy calls in removal blocks
.changeset/eight-ears-feel.md	Documents the fix for Tree.key and Tree.parent for unhydrated nodes

Comments suppressed due to low confidence (1)

packages/dds/tree/src/simple-tree/core/unhydratedFlexTree.ts:503

• Since the explicit adoptBy(undefined) call for removed nodes was removed in favor of centralized reset logic in the edit() method, please add a comment clarifying this change to improve maintainability.

const c = this.mapTrees[i];

[Josmithr]

Approving for docs. The code changes seem reasonable to me, but I'm missing some context. Would probably be good to get a review from @noencke or someone too.