refactor: clean up LGraphCanvas.cloneZIndex.test.ts (follow-up from #10361)

## Follow-up cleanup for `LGraphCanvas.cloneZIndex.test.ts`

Tracked from review comments in #10361.

### 1. Move describe-level mutable state into individual `it` blocks

**Comment by DrJKL:** [link](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10361#discussion_r3047062466)

The `graph`, `canvas`, and `previousVueNodesMode` variables are declared at `describe` scope and mutated in `beforeEach`. DrJKL flagged this as a cross-test contamination risk even with two tests — each `it` block should instantiate its own local variables rather than sharing mutable describe-level state.

### 2. Extract `createCanvas` into a shared test utility

**Comment by DrJKL:** [link](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10361#discussion_r3019705055)

The `createCanvas` helper in this test file is duplicated from `LGraphCanvas.groupSelection.test.ts`. It should be extracted into a shared test utility to avoid copy-paste drift.

### 3. Replace or improve the bespoke `TestNode` class

**Comment by DrJKL:** [link](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10361#discussion_r3047072468)

A custom `TestNode` class is defined inline for the test. Prefer either:
- Using a real registered node type instead of a bespoke class, or
- If `TestNode` is kept, update its constructor to accept all setup options (position, size) directly so post-instantiation mutation is not required.

### 4. Remove unnecessary type casts

**Comment by DrJKL:** [link](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10361#discussion_r3047075122)

The cloned nodes are cast as `as LGraphNode` in the test assertions. These casts should either be removed if the types are already correct, or the reason should be documented if they are genuinely needed.

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Backlink: PR #10361 (https://github.com/Comfy-Org/ComfyUI_frontend/pull/10361)
Requested by: @dante01yoon

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11169-refactor-clean-up-LGraphCanvas-cloneZIndex-test-ts-follow-up-from-10361-33f6d73d36508103bbfcf6261dffe704) by [Unito](https://www.unito.io)