The post editor uses the permalink_template property from the posts REST API to render the post slug input or not. For auto-drafts or newly created posts, this property was not set properly. This PR solves by fixing the get_sample_permalink function used to compute this property.
Props youknowriad, mcsf, antonvlasenko, azaozz, peterwilsoncc, andrewserong, hellofromTonya, spacedmonkey.
Fixes#59283.
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