refactor(tree): Leverage schema type-narrowing helper functions

[Josmithr]

Replaces some explicit node.kind equality checks with existing type-narrowing helpers.

[Copilot]

Pull Request Overview

This PR refactors schema-kind checks by leveraging existing type-narrowing helper functions instead of direct node.kind comparisons.

• Replaced all explicit schema.kind === NodeKind.* checks with is*NodeSchema(schema) and added corresponding assert calls.
• Updated function signatures (arrayToFlexContent, mapToFlexContent, objectToFlexContent) to accept specific schema types.
• Consolidated imports of schema type guards and disabled internal-module linting around them.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
packages/dds/tree/src/simple-tree/unhydratedFlexTreeFromInsertable.ts	Swapped kind checks for is*NodeSchema asserts; tightened helper signatures and reorganized imports
packages/dds/tree/src/simple-tree/api/treeNodeApi.ts	Changed array-kind branch to use isArrayNodeSchema instead of NodeKind.Array
packages/dds/tree/src/simple-tree/api/customTree.ts	Replaced NodeKind.Array check with isArrayNodeSchema
packages/dds/tree/src/simple-tree/api/configuration.ts	Updated union validation to use isArrayNodeSchema/isMapNodeSchema and refined typed arrays/maps