Revisions Slider: Add aria-valuetext for screen reader accessibility

[R1shabh-Gupta]

What?

Part of #77530

Slider: Visual label not available to screen readers. Use aria-valuetext to allow screen readers to get the text interpretation of the numeric revision value.

Adds aria-valuetext to the revisions slider so screen readers announce a meaningful string instead of a bare numeric index.

Why?

The revisions slider uses a 0-based index as its value. Screen readers would announce "3" with no context, giving users no useful information about which revision they're on.

How?

Computes a human-readable string from the currently selected revision, e.g. "Revision 3 of 7: January 15, 2025 at 2:30 pm" and passes it as aria-valuetext on the RangeControl.

Testing Instructions

1. Open a post or page that has at least two saved revisions.
2. Click the Revisions button in the right side toolbar.
3. Inspect the slider element in DevTools — the <input type="range"> should have an aria-valuetext attribute.
4. Move the slider left/right and confirm aria-valuetext updates to reflect the newly selected revision's number and date.

Testing Instructions for Keyboard

1. Open a post with multiple revisions and navigate to the Revisions panel.
2. Tab to the revisions slider.
3. Enable a screen reader (e.g. VoiceOver on macOS: Cmd+F5).
4. Press the left/right arrow keys to move through revisions and the screen reader should announce the full revision label (e.g. "Revision 3 of 7: April 21, 2026 at 3:45 pm") instead of just a number.

Screenshots or screencast

Before

Before-77530.mp4

After

After-77530.mp4

Use of AI Tools

I used Claude Code to review the code.

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

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