Slideshow block assigns role="group" to <li> slides, breaking list semantics

> Lists do not contain only `

## Current behavior

After initialization, the rendered markup includes slides similar to:

```html
<ul class="wp-block-jetpack-slideshow_swiper-wrapper">
<li class="wp-block-jetpack-slideshow_slide swiper-slide" role="group">
<!-- Slide content -->
</li>
</ul>
```

The resulting accessibility structure is interpreted as:

```text
list
└── group
```

## Expected behavior

Slides rendered as `<li>` elements should preserve their `listitem` semantics:

```text
list
└── listitem
```

## Possible solution

Swiper supports configuring the role assigned to each slide:

```js
a11y: {
slideRole: 'listitem',
}
```

A possible solution may be to set `a11y.slideRole` to `listitem` when initializing Slideshow blocks whose slides are rendered as `<li>` elements.

WordPress VIP Support confirmed that Jetpack currently initializes Swiper without setting this option, so Swiper falls back to its default `group` role. They also confirmed that there is currently no supported PHP filter or JavaScript hook for overriding this configuration from application code.

I’d be happy to prepare a pull request if this approach looks appropriate.

### Steps to reproduce

1. Create a post or page.
2. Add a Jetpack Slideshow block containing multiple images.
3. Publish the page and open it on the frontend.
4. Inspect one of the slideshow items after Swiper initializes.
5. Observe that the `<li>` element has `role="group"`.
6. Run an accessibility audit using axe or PageSpeed Insights.

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Moderate

### What other impact(s) does this issue have?

_No response_

### If a workaround is available, please outline it here.

A temporary application-level workaround is available using custom JavaScript.

The script waits for the Jetpack Slideshow Swiper instance to initialize, then:

- sets `swiper.params.a11y.slideRole` to `"listitem"`;
- updates `swiper.originalParams.a11y.slideRole` as well;
- changes existing slide elements from `role="group"` to `role="listitem"`;
- runs only when a Jetpack Slideshow block is present.

This restores the expected list semantics for the `<li>` slide elements. However, it is only a compatibility workaround and the preferred fix would be to set the correct `slideRole` directly in Jetpack’s Swiper configuration.

### Platform (Simple and/or Atomic)

Atomic