Revisions: Add tooltip to diff marker buttons

[himanshupathak95]

What?

Part of #77530

Replaces the raw <button> elements in the diff markers minimap with the <Button> component, adding label and showTooltip props.

Why?

Diff marker buttons had no visible tooltip, and sighted users hovering the minimap received no textual information about what the coloured bar represents.

How?

label on <Button> sets aria-label and renders visible text when the user has "text only buttons" enabled. showTooltip ensures the Gutenberg <Tooltip> always appears on hover with the same label text ("Go to added block", "Go to removed block", "Go to modified block").

Testing Instructions

1. Create a post with multiple revisions and enter the visual revisions view.
2. Enable Show changes (eye icon). Coloured bars appear in the right-hand minimap.
3. Hover any bar, and a tooltip should appear with the text "Go to added block", "Go to removed block", or "Go to modified block".
4. Verfiy the aria-labels stay intact and as expected.

Screenshots or screencast

Before

Screen.Recording.2026-04-28.at.10.37.34.mov

After

Screen.Recording.2026-04-28.at.10.51.30.mov

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