fix(send): use stored WECOM_BOT webhook URL and preserve single-chat routing without scene

- Copy account.ext_info into extra.webhook_url for WECOM_BOT sends.
- Allow null conversation_scene for single chats; only enforce scene-based routing for group chats.
- Harden WeComBotSender custom URL handling when placeholder is missing.

backend/mci-channel-wecom/src/main/java/com/sino/mci/channel/wecom/WeComBotSender.java

@@ -85,7 +85,11 @@ public class WeComBotSender {
     private String resolveWebhookUrl(Map<String, Object> extra, String key) {
         String customUrl = getString(extra, "webhook_url");
         if (customUrl != null && !customUrl.isBlank()) {
-            return String.format(customUrl, key);
+            if (customUrl.contains("%s")) {
+                return customUrl.replace("%s", key);
+            }
+            log.warn("[WeComBot] custom webhook_url missing %s placeholder; appending key param");
+            return customUrl.contains("?") ? customUrl + "&key=" + key : customUrl + "?key=" + key;
         }
         return String.format(DEFAULT_WEBHOOK_URL, key);
     }