Media: Fix strpos() check and file_get_contents() handling in wp_get_…

[tejas0306]

Fixes https://core.trac.wordpress.org/ticket/64849#ticket

Summary

The wp_get_image_alttext() function in src/wp-admin/includes/image.php checks for XMP metadata using strpos(), but the current implementation uses a loose comparison:

if ( ! $xmp_start || ! $xmp_end ) {
    return $alt_text;
}

[github-actions]

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Props suhan2411.

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62027
GitHub commit: 6dac3b4

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.