sources: fix dummy names being suggested as other names.

Fix placeholder names like weibo_1234, bilibili_1234, nijie1234, and nicoseiga1234 being suggested
as Other Names when creating new artist entries. These are meant to be placeholders for the tag
name, not used as other names.

app/logical/source/extractor/nico_seiga.rb

@@ -40,7 +40,7 @@ module Source
       end
 
       def profile_url
-        "https://seiga.nicovideo.jp/user/illust/#{api_client.user_id}" if api_client&.user_id.present?
+        "https://seiga.nicovideo.jp/user/illust/#{artist_id}" if artist_id.present?
       end
 
       def artist_name
@@ -67,8 +67,11 @@ module Source
       end
 
       def tag_name
-        return if api_client&.user_id.blank?
-        "nicoseiga#{api_client.user_id}"
+        "nicoseiga_#{artist_id}" if artist_id.present?
+      end
+
+      def other_names
+        [artist_name].compact
       end
 
       def tags
@@ -95,6 +98,10 @@ module Source
         parsed_url.manga_id || parsed_referer&.manga_id
       end
 
+      def artist_id
+        api_client&.user_id
+      end
+
       def http
         if parsed_url.oekaki_id.present?
           super.with_legacy_ssl