MDEV-30977: Additional key values are not validating properly when using

unevaluatedProperties with properties declared in subschemas

Analysis:
When a key fails to validate for "properties" when "properties" is being
treated as alternate schema, it needs to fall back on alternate schema
for "properites" itself ("unevaluatedProperties" in context of the bug).
But that doesn't happen and we end up returning false (=validated)
Fix:
When "properties" fails to validate as an alternate schema, fall back on
alternate schema for "properties" itself.

sql/json_schema.cc

@@ -1883,6 +1883,13 @@ bool Json_schema_properties::validate_as_alternate(const json_engine_t *je,
         return true;
     }
   }
+  else
+  {
+    if (alternate_schema && alternate_schema->validate_as_alternate(je, k_start, k_end))
+    {
+      return true;
+    }
+  }
   return false;
 }
 
@@ -1892,6 +1899,8 @@ Json_schema_additional_and_unevaluated::
                                                      const uchar* k_start,
                                                      const uchar* k_end)
 {
+  if (!allowed)
+    return true;
   return validate_schema_items(je, &schema_list);
 }