[compute logs] fix subscription (#28110)

* we were exiting with stuff still in the queue since we were just
checking the subscription before exiting
* we were blocking main thread with downloads since we do a `fetch` on
`subscribe`

## How I Tested These Changes

existing coverage, look compute logs in `dagster dev` and dagster+

python_modules/dagster-graphql/dagster_graphql/implementation/execution/__init__.py

@@ -321,13 +321,15 @@ async def gen_captured_log_data(
     def _enqueue(new_event):
         loop.call_soon_threadsafe(queue.put_nowait, new_event)
 
-    subscription(_enqueue)
+    # subscription object will attempt to fetch when started, so move off main thread
+    await run_in_threadpool(subscription, _enqueue)
+
     is_complete = False
     try:
         while not is_complete:
             update = await queue.get()
             yield from_captured_log_data(update)
-            is_complete = subscription.is_complete
+            is_complete = subscription.is_complete and queue.empty()
     finally:
         subscription.dispose()