fix(model): alias JsonRpcMessageVisitor PhantomData for clippy

Factor the PhantomData<fn() -> (Req, Resp, Noti)> marker into a type
alias so clippy::type_complexity no longer fails CI under -D warnings.

Author: ump45nose

crates/rmcp/src/model.rs

@@ -862,8 +862,11 @@ where
     }
 }
 
+type JsonRpcMessageVisitorMarker<Req, Resp, Noti> =
+    std::marker::PhantomData<fn() -> (Req, Resp, Noti)>;
+
 struct JsonRpcMessageVisitor<Req, Resp, Noti> {
-    _marker: std::marker::PhantomData<fn() -> (Req, Resp, Noti)>,
+    _marker: JsonRpcMessageVisitorMarker<Req, Resp, Noti>,
 }
 
 impl<'de, Req, Resp, Noti> serde::de::Visitor<'de> for JsonRpcMessageVisitor<Req, Resp, Noti>