Make LocateNodeButton keyboard activation test focus the button explicitly

## Summary

Make the `LocateNodeButton` keyboard-activation test explicitly focus the rendered button before it sends the `Enter` key.

## Rationale

The current test uses `user.tab()` and relies on the JSDOM implicit tab order. The test has a single focusable element today, but an additional focusable element in the PrimeVue button structure could cause the test to activate the wrong element.

## Affected area

- `src/components/rightSidePanel/errors/LocateNodeButton.test.ts`
- The `emits locate on keyboard activation` test

## Required change

Get the button by its accessible role and name, call `.focus()` on it, then send `{Enter}` with `user.keyboard()`.

## Acceptance criteria

- The test explicitly focuses the `LocateNodeButton` before keyboard activation.
- Pressing `Enter` emits `locate` exactly once.
- The test continues to select the button by its accessible role and name.

## Context

Requested by @christian-byrne as a non-blocking follow-up from pull request #13401.

- Pull request: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13401
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13401#discussion_r3634946910