fix(send-routing): address re-review issues

- Set conversationScene=EXTERNAL in SendEndToEndTest and align account to WECHAT_PERSONAL
- Replace literal account type code with AccountType.WECHAT_PERSONAL.getCode()
- Use replaceFirst for webhook URL placeholder in WeComBotSender
- Remove unused selectDefaultAccount overload
- Hide scene selector for single-chat conversations in admin-web

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

@@ -91,7 +91,7 @@ public class WeComBotSender {
         String customUrl = getString(extra, "webhook_url");
         if (customUrl != null && !customUrl.isBlank()) {
             if (customUrl.contains("%s")) {
-                return customUrl.replace("%s", key);
+                return customUrl.replaceFirst("%s", key);
             }
             return customUrl;
         }