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

NEWS

@@ -143,6 +143,12 @@ CHANGES WITH 262 in spe:
           "none" returned previously. In the human-readable table output a
           hierarchy that is not merged is now shown as "-" instead of "none".
 
+        * The JSON payload returned by systemd-sysupdated's DescribeFeature()
+          method changed the "name" and "enabled" keys to "id" and
+          "isEnabled", respectively. The "appStream" key is now "appstream",
+          and the "documentation" key is now an array of strings. Consumers
+          of this interface must update their JSON parsing accordingly.
+
         Changes in the system and service manager:
 
         * The manager now embeds a basic set of unit files (basic.target,