[53218] introduced a bug of a wrong response from the `wp/v2/pattern-directory/patterns` endpoint with a `slug` parameter. As the response is cached, it can result in an incorrect list of available patterns supported by the current theme.
This commit resolves by:
* Limiting the `slug` to an `array` in the query parameters.
* When set, parsing and sorting the slug(s) and then serializing the sorted query args as part of the hashed transient keys.
Props antonvlasenko, timothyblynjacobs, spacedmonkey, costdev, hellofromTonya.
Follow-up to [53218], [53152], [51208].
Fixes#55617.
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