[lexical][*] Chore: stop using deprecated traversal type parameters in tests

[etrepum]

Description

Migrate the remaining call sites (all in tests) off the deprecated unsafe type parameters that #8661 added to the node traversal methods (getFirstChild(), getParent(), getChildAtIndex(), $getNodeByKey(), etc.). The deprecated overloads themselves are retained for backwards compatibility with consumers.

Add a $assertNodeType test helper that narrows a node via a type guard instead of an unchecked cast, and use it (alongside the existing $is* guards) in place of the deprecated type parameters and a few remaining unsafe as casts in the affected tests.

Follow-up to #8661

Test plan

Only unit test syntax is affected, behavior should be identical and no runtime code has changed.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Jun 10, 2026 1:55pm
lexical-playground	Ready	Preview, Comment	Jun 10, 2026 1:55pm

[potatowagon]

Reviewed by Navi (Tater Thoughts Bobblehead) on behalf of @potatowagon.

Assessment: LGTM ✅

This is a well-scoped refactor that replaces deprecated generic type parameters on traversal methods (e.g. getFirstChildOrThrow<T>(), getFirstChild<T>(), getChildAtIndex<T>()) with a new runtime $assertNodeType() utility that uses type guards for type-safe narrowing.

What I checked:

1. The $assertNodeType utility (packages/lexical/src/__tests__/utils/index.tsx): Clean implementation — takes a node | null | undefined and a type guard, throws a clear error if the guard fails. Returns the narrowed type. This is strictly better than the previous as T / ! pattern because it provides a runtime assertion rather than silently producing undefined behavior on type mismatch.

2. Consistency of the migration pattern across all 21 test files: Every change follows the same mechanical pattern — getFirstChildOrThrow<Foo>() → $assertNodeType(getFirstChild(), $isFoo), and getChildAtIndex<Foo>(n)! → $assertNodeType(getChildAtIndex(n), $isFoo). No logic changes, just type narrowing approach.

3. Import cleanup: Removed unused concrete type imports (e.g. ParagraphNode, TextNode, TableNode) and added the corresponding type guard imports ($isParagraphNode, $isTextNode, $isTableNode). This is correct — the concrete types are no longer needed when using guards.

4. Edge cases: The utility correctly handles null | undefined by coalescing to null before passing to the guard. One creative use in LexicalReconciler.test.ts uses a custom inline guard (node): node is TestDecoratorNode => node instanceof TestDecoratorNode — this is valid and shows the utility is flexible.

5. www backwards compatibility: This PR only touches test files and a test utility. No production API changes, no export changes, no signature changes. Zero www impact.

6. CI: Core unit tests (node 24.x) and core browser tests pass. CLA signed. Vercel previews deployed. Browser/e2e/integration tests still pending but those are test-only changes that should not regress.

Why this is safe: All changes are test-only. The deprecated generic type parameters still work in production code — this just migrates tests away from them so the deprecation can proceed. No runtime behavior change in any library package.