Image: Fix missing aria-label on lightbox trigger button for single images

[R1shabh-Gupta]

What?

Closes #78413

Restores the aria-label attribute on the lightbox trigger button for single image blocks that have "Enlarge on click" enabled.

Why?

The commit that added gallery lightbox support changed the trigger button to use a dynamic aria-label binding, but left triggerButtonAriaLabel as null for single images. So the attribute was never rendered on the
frontend.

How?

Set triggerButtonAriaLabel to "Enlarge" at render time in the image block's PHP render function.

Testing Instructions

1. Create a new post or page.
2. Insert a single Image block and upload or select any image.
3. With the image selected, open the Link toolbar dropdown and choose "Enlarge on click".
4. Publish or preview the page.
5. Inspect the lightbox trigger button (button.lightbox-trigger) in browser DevTools and confirm it has aria-label="Enlarge".

Testing Instructions for Keyboard

1. On the published/previewed page, Tab to the lightbox trigger button on the image.
2. Activate VoiceOver (macOS: Cmd + F5) or NVDA (Windows).
3. Confirm the screen reader announces "Enlarge".

Screenshots or screencast

Before	After

Use of AI Tools

None

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Co-authored-by: R1shabh-Gupta <rishabhwp@git.wordpress.org>
Co-authored-by: joedolson <joedolson@git.wordpress.org>

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[t-hamano]

Nice catch!