Add aria-label to Revisions button in Post Summary sidebar

[vishnucmsminds]

What?

Closes #78138

This PR adds a descriptive aria-label to the Revisions button in the Post Summary sidebar panel.

Why?

Previously, the Revisions button in the Post Summary sidebar only displayed the count of revisions (e.g., "3"). For screen reader users, this was announced simply as "3 button," which lacked context. Adding a descriptive aria-label ensures that users understand the button's purpose is to open the revisions screen.

How?

In packages/editor/src/components/post-last-revision/index.js, I've updated the PrivatePostLastRevision component's Button to include an ariaLabel prop. The label is localized and dynamically includes the revision count: Open revisions screen: %s revisions.

Testing Instructions

1. Open a post or page that has at least two revisions.
2. Open the Settings sidebar and ensure you are on the Post tab.
3. Locate the Summary panel.
4. Inspect the Revisions button (the one showing the number of revisions).
5. Verify that it now has an aria-label attribute like aria-label="Open revisions screen: X revisions".

Testing Instructions for Keyboard

1. Navigate to the Revisions button using the Tab key.
2. Use a screen reader (like VoiceOver or NVDA) to verify that the button is announced with the new descriptive label instead of just the number.

Use of AI Tools

This PR was authored with the assistance of Claude Opus 4.6. I have reviewed and tested the changes to ensure they meet the project's standards and solve the reported accessibility issue.

[github-actions]

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Co-authored-by: vishnupprajapat <vishnuprajapat@git.wordpress.org>
Co-authored-by: vishnucmsminds <prajapatvishnu@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[github-actions]

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[t-hamano]

LGTM!

[t-hamano]

@vishnucmsminds, Do you know about E2E tests? The button label has changed, so we need to update the E2E tests accordingly. The files that need to be modified are as follows:

• specs/editor/various/footnotes-revisions.spec.js
• specs/editor/various/revision-block-diff.spec.js
• specs/editor/various/revision-fields-diff.spec.js

In other words, we need to change the code included in the file above as follows.

                } );
                await settingsSidebar.getByRole( 'tab', { name: 'Post' } ).click();
                await settingsSidebar
-                       .getByRole( 'button', { name: '2', exact: true } )
+                       .getByRole( 'button', { name: 'Open revisions screen: 2 revisions' } )
                        .click();
                await expect(
                        page.getByRole( 'button', { name: 'Restore' } )

[vishnucmsminds]

• specs/editor/various/footnotes-revisions.spec.js
• specs/editor/various/revision-block-diff.spec.js
• specs/editor/various/revision-fields-diff.spec.js

Thanks for the feedback @t-hamano! I've updated the E2E tests in the following files to match the new button label:
test/e2e/specs/editor/various/footnotes-revisions.spec.js
test/e2e/specs/editor/various/revision-block-diff.spec.js
test/e2e/specs/editor/various/revision-fields-diff.spec.js

Please let me know if any further changes are needed.

[github-actions]

I just cherry-picked this PR to the wp/7.0 branch to get it included in the next release: ad384d5