fix: ADD SWITCHER keyword - use register_custom_syntax instead of register_fn

The compiler outputs ADD_SWITCHER "x" as "y" format but the keyword
was registered as a plain Rhai function expecting add_switcher(x, y).
Changed to register_custom_syntax matching the output format, consistent
with ADD_SUGGESTION and CLEAR SWITCHERS patterns.

Fixes #495

botserver/src/basic/keywords/switcher.rs

@@ -92,15 +92,24 @@ pub fn add_switcher_keyword(
     let user_session_clone = user_session.clone();
 
     engine
-  .register_fn("add_switcher", move |switcher_id: &str, button_text: &str| {
+        .register_custom_syntax(
-    let result = add_switcher(
+            ["ADD_SWITCHER", "$expr$", "as", "$expr$"],
+            true,
+            move |context, inputs| {
+                let switcher_id = context.eval_expression_tree(&inputs[0])?.to_string();
+                let button_text = context.eval_expression_tree(&inputs[1])?.to_string();
+
+                add_switcher(
                     cache.as_ref(),
                     &user_session_clone,
-      switcher_id,
+                    &switcher_id,
-      button_text,
+                    &button_text,
-    );
+                )?;
-    result.map_err(|e| e.to_string())
+
-  });
+                Ok(Dynamic::UNIT)
+            },
+        )
+        .expect("valid syntax registration");
 }
 
 fn add_switcher(