Add featured image alt text

[prajapatisagar]

Addresses part of #66005

What?

This PR ensures that when an image fails to load (i.e., is broken), the alt text is displayed in its place. If alt text is not provided, the image name will be displayed instead.

Why?

This update improves accessibility and enhances user experience by providing context when an image fails to load. Instead of leaving a blank space or displaying a broken image icon, the alt text or image name will be shown, offering users helpful information.

How?

The implementation involves updating the image rendering logic to check if the image fails to load. If it does, the alt text (if available) will be displayed. If the alt text is not provided, the image name will be used as a fallback display.

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: prajapatisagar <sagarprajapati@git.wordpress.org>
Co-authored-by: afercia <afercia@git.wordpress.org>

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[afercia]

Thanks @prajapatisagar !
The same code is in use for the text used for the aria-describedby. To avoid code duplication, I'd suggest to abstrat it if possible to a small helper function and use that function for both the aria-describedby and the alt attribute.

[afercia]

Thanks @prajapatisagar LGTM.