updatectl: fix feature JSON parsing

The feature JSON payload switched to `id`, `isEnabled`, and `appstream`,
and made `documentation` an array of URLs.
`updatectl` still parsed legacy table-derived names and a single URL.
This broke feature JSON parsing and made output inconsistent.
Features without documentation could also cause a null dereference.

Align `updatectl` with the current schema and preserve all
documentation URLs across sysupdate, D-Bus, and Varlink payloads.
Propagate condition evaluation errors through `condition_test_list()`
and `feature_is_suggested()`, with consistent JSON error handling and
tests.
The `suggested` field is now optional and always a boolean when
present; it is omitted rather than set to an error string when
SuggestOn*= conditions cannot be evaluated.
Add `suggested` metadata and document the field transition.

Follow-up for 3e34ccf6df.
Follow-up for 75a247d4de.
Follow-up for 40e727c93b.

Fixes #43603

man/sysupdate.components.xml

@@ -98,8 +98,9 @@
       <varlistentry>
         <term><varname>Documentation=</varname></term>
 
-        <listitem><para>A user-presentable URL to documentation about this component.
-        This setting supports specifier expansion; see below for details on supported specifiers.</para>
+        <listitem><para>One or more user-presentable URLs to documentation about this component. This setting may be
+        specified more than once. If the empty string is assigned to this option, the list is reset, and all prior
+        assignments have no effect. It supports specifier expansion; see below for details on supported specifiers.</para>
 
         <xi:include href="version-info.xml" xpointer="v262"/></listitem>
       </varlistentry>