persist: fix incorrect "dangling blob keys" WARN

In MaterializeInc#21237, I changed how the `into_transmittable_batch` serialization
worked and lost the mark_consumed call. This results in persist
incorrectly concluding that the batch had been dropped without being
processed in some way, and thus incorrectly warning about it.

src/persist-client/src/batch.rs

@@ -177,12 +177,14 @@ where
     /// dropped, the data that it represents will have leaked. The caller is
     /// responsible for turning this back into a [`Batch`] using
     /// [`WriteHandle::batch_from_transmittable_batch`](crate::write::WriteHandle::batch_from_transmittable_batch).
-    pub fn into_transmittable_batch(self) -> ProtoBatch {
-        ProtoBatch {
+    pub fn into_transmittable_batch(mut self) -> ProtoBatch {
+        let ret = ProtoBatch {
             shard_id: self.shard_id.into_proto(),
             version: self.version.to_string(),
             batch: Some(self.batch.into_proto()),
-        }
+        };
+        self.mark_consumed();
+        ret
     }
 }