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

src/shared/varlink-io.systemd.SysUpdate.c

@@ -42,6 +42,8 @@ static SD_VARLINK_DEFINE_STRUCT_TYPE(
                 SD_VARLINK_DEFINE_FIELD(appstream, SD_VARLINK_STRING, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("Whether the feature is enabled."),
                 SD_VARLINK_DEFINE_FIELD(isEnabled, SD_VARLINK_BOOL, 0),
+                SD_VARLINK_FIELD_COMMENT("Whether the feature is suggested for enablement."),
+                SD_VARLINK_DEFINE_FIELD(suggested, SD_VARLINK_BOOL, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("Array of IDs of the transfers (including currently disabled ones) which are controlled by this feature."),
                 SD_VARLINK_DEFINE_FIELD(transfers, SD_VARLINK_STRING, SD_VARLINK_NULLABLE|SD_VARLINK_ARRAY));