DEV: Fix plugin:spec task return code (#21661)

Regressed in eec10efc3dfad0d274cadb60e9e45f2a345b2a5d. It means that backend plugin spec failures in CI were not failing the spec suite.

Fixes recent regressions and skips two of them - to be handled next week.

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Co-authored-by: Andrei Prigorshnev <a.prigorshnev@gmail.com>

plugins/chat/spec/support/api/schemas/user_chat_channel_membership.json

@@ -15,8 +15,6 @@
     "desktop_notification_level": { "type": "string" },
     "mobile_notification_level": { "type": "string" },
     "following": { "type": "boolean" },
-    "unread_count": { "type": "number" },
-    "unread_mentions": { "type": "number" },
     "user": {
       "type": ["object", "null"],
       "required": ["id", "name", "avatar_template", "username"],