Investigate simplification of `promptRenameWidget` in `widgetUtil.ts`

## Overview

Investigate whether the `promptRenameWidget` function in `src/utils/widgetUtil.ts` can be further simplified.

## Context

This was flagged during the review of #10245 (fix: App mode - renaming widgets on subgraphs) by @pythongosssss.

## Current Implementation

The function currently (lines 96–113 of `src/utils/widgetUtil.ts`):

1. Prompts the user for a new widget label via `promptWidgetLabel`.
2. Returns `null` if the prompt was cancelled or the input is empty after trimming.
3. Delegates to `renameWidget` to apply the rename (handling subgraph promotion and input propagation).
4. Fires `widget.callback` and marks the canvas dirty.

## Investigation Points

- Can the `null`-check / early-return logic be collapsed or moved into a shared utility?
- Can `renameWidget` absorb the canvas-dirty / callback side effects, removing them from `promptRenameWidget`?
- Are there other call sites that duplicate parts of this logic that could be unified?
- Consider whether the `parents` parameter handling (now broadened to include subgraph nodes without explicit parents as of #10245) can be made more ergonomic.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10245
- File: `src/utils/widgetUtil.ts`
- Requested by: @pythongosssss

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