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/sysupdate/sysupdate-feature.h

@@ -9,7 +9,7 @@ typedef struct Feature {
         char *id;
 
         char *description;
-        char *documentation;
+        char **documentation;
         char *appstream;
 
         bool enabled;
@@ -27,4 +27,4 @@ extern const struct hash_ops feature_hash_ops;
 
 int feature_read_definition(Feature *f, const char *root, const char *path, const char *const *conf_file_dirs);
 
-int feature_is_suggested(Feature *f);
+int feature_is_suggested(const Feature *f);