Fix #reply{} return when requesting an existing manifest

The manifest machine `apply/3` function should always return a list
of effects. This fixes `rabbitmq_stream_s3_log_manifest:get_manifest/1`
when the manifest is already downloaded / cached in memory.

Author: the-mikedavis

src/rabbitmq_stream_s3_log_manifest_machine.erl

@@ -222,7 +222,7 @@ apply(
 ) ->
     case Manifests0 of
         #{Dir := {#manifest{} = Manifest, _UploadStatus}} ->
-            {State0, #reply{to = Requester, response = Manifest}};
+            {State0, [#reply{to = Requester, response = Manifest}]};
         #{Dir := {pending, Requesters}} ->
             Manifests = Manifests0#{Dir := {pending, [Requester | Requesters]}},
             {State0#?MODULE{manifests = Manifests}, []};