Add missing id attribute to RCA AI candidate action to_config()

With this missing, certain combinations of actions/events would lead to
the ids of all CAs being dropped (e.g. in saves after using
[modify_ai]), which resulted in it not being possible to remove CAs any
more as that is done by id.
This fixes bug #21750.

changelog

@@ -1,4 +1,9 @@
 Version 1.11.11+dev:
+ * AI:
+   * Fixed a bug that made it impossible to change or delete Micro AIs after a
+     game had been reloaded (bug #21750). This was a general bug in the RCA AI
+     mechanism and applied to other CAs as well, but it was most visible in the
+     Micro AIs.
  * Language and i18n:
    * Updated translations: German
 

src/ai/composite/rca.cpp

@@ -90,6 +90,7 @@ config candidate_action::to_config() const
 	config cfg;
 	cfg["enabled"] = enabled_;
 	cfg["engine"] = engine_;
+	cfg["id"] = id_;
 	cfg["name"] = name_;
 	cfg["score"] = score_;
 	cfg["max_score"] = max_score_;