feat: implement remote unfollow — wire FederationActionPort through delete_follow handler

crates/domain/src/testing.rs

@@ -548,6 +548,14 @@ impl FederationActionPort for TestStore {
         Ok(())
     }
 
+    async fn unfollow_remote(
+        &self,
+        _local_user_id: &UserId,
+        _handle: &str,
+    ) -> Result<(), DomainError> {
+        Ok(())
+    }
+
     async fn actor_json(&self, _user_id: &UserId) -> Result<String, DomainError> {
         Err(DomainError::NotFound)
     }