Refactor handling of ContextServer notifications

The notification handler registration is now more explicit, with
handlers set up before server initialization to avoid potential race
conditions.

crates/context_server/src/protocol.rs

@@ -115,10 +115,11 @@ impl InitializedContextServerProtocol {
         self.inner.notify(T::METHOD, params)
     }
 
-    pub fn on_notification<F>(&self, method: &'static str, f: F)
-    where
-        F: 'static + Send + FnMut(Value, AsyncApp),
-    {
+    pub fn on_notification(
+        &self,
+        method: &'static str,
+        f: Box<dyn 'static + Send + FnMut(Value, AsyncApp)>,
+    ) {
         self.inner.on_notification(method, f);
     }
 }