feat(subsonic): add OS readonly and validUntil properties in playlists (#4993)

* feat(subsonic): add OS readonly and validUntil properties

* remove duplicated test

* test: fix and enable disabled child smart playlist tests

Fixed the XContext("child smart playlists") tests that were disabled with
a TODO comment. The tests had several issues: nested playlists were missing
Public: true (required by InPlaylist criteria), the criteria matched no
test fixtures, the "not expired" test set EvaluatedAt on the parent too
(preventing it from refreshing at all), and the "expired" test dereferenced
a nil EvaluatedAt. Added proper cleanup with DeferCleanup and config
restoration via configtest.

* fix(subsonic): always include readonly field in JSON playlist responses

Removed omitempty from the JSON tag of the Readonly field in
OpenSubsonicPlaylist so that readonly: false is always serialized in
JSON responses, per the OpenSubsonic spec requirement that supported
fields must be returned with default values. Added a test case with an
empty OpenSubsonicPlaylist to verify the behavior.

---------

Co-authored-by: Deluan Quintão <deluan@navidrome.org>

server/subsonic/responses/.snapshots/Responses Playlists with data should match .JSON

@@ -14,9 +14,20 @@
         "duration": 120,
         "public": true,
         "owner": "admin",
+        "created": "2023-02-20T14:45:00Z",
+        "changed": "2023-02-20T14:45:00Z",
+        "coverArt": "pl-123123123123",
+        "readonly": true,
+        "validUntil": "2023-02-20T14:45:00Z"
+      },
+      {
+        "id": "333",
+        "name": "ccc",
+        "songCount": 0,
+        "duration": 0,
         "created": "0001-01-01T00:00:00Z",
         "changed": "0001-01-01T00:00:00Z",
-        "coverArt": "pl-123123123123"
+        "readonly": false
       },
       {
         "id": "222",

server/subsonic/responses/.snapshots/Responses Playlists with data should match .XML

@@ -1,6 +1,7 @@
 <subsonic-response xmlns="http://subsonic.org/restapi" status="ok" version="1.16.1" type="navidrome" serverVersion="v0.55.0" openSubsonic="true">
   <playlists>
-    <playlist id="111" name="aaa" comment="comment" songCount="2" duration="120" public="true" owner="admin" created="0001-01-01T00:00:00Z" changed="0001-01-01T00:00:00Z" coverArt="pl-123123123123"></playlist>
+    <playlist id="111" name="aaa" comment="comment" songCount="2" duration="120" public="true" owner="admin" created="2023-02-20T14:45:00Z" changed="2023-02-20T14:45:00Z" coverArt="pl-123123123123" readonly="true" validUntil="2023-02-20T14:45:00Z"></playlist>
+    <playlist id="333" name="ccc" songCount="0" duration="0" created="0001-01-01T00:00:00Z" changed="0001-01-01T00:00:00Z"></playlist>
     <playlist id="222" name="bbb" songCount="0" duration="0" created="0001-01-01T00:00:00Z" changed="0001-01-01T00:00:00Z"></playlist>
   </playlists>
 </subsonic-response>