Media: Guard against false return from wp_get_attachment_image_src() for PHP 8.5

[roshniahuja]

Summary

• Fixes PHP 8.5 warning when list() is used to destructure false returned by wp_get_attachment_image_src()
• Adds is_array() guards in embed.php (get_oembed_response_data_rich), and media.php (wp_get_attachment_image_src, wp_playlist_shortcode, wp_prepare_attachment_for_js)
• Adds unit tests for invalid thumbnail scenarios

Trac Ticket

https://core.trac.wordpress.org/ticket/64742

Test Plan

• PHPUnit tests pass (430 tests in Media + oEmbed suites)
• Manual test: set _thumbnail_id to a non-existent ID, visit oEmbed endpoint — no warnings, no null thumbnail keys

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[westonruter]

Thanks for the PR, but there is an existing one linked on the ticket which is nearing merge. See #11073.