Fix flake on test_path_temporary_files_are_removed (#2059)

I was seeing this test fail in CI (example runs below).  It turns out this test
has a race condition - the test asserts that the temp file has been deleted, but
the temp file is deleted asynchronously on a
concurrent.futures.Future.add_done_callback() callback.  There is no way to ask
a Future to wait until callbacks are finished running (without having the
callback itself perform some sort of coordination, which feels like overkill for
our needs here).

I fixed this with a quick bodge - we test if the file has been deleted, but if
it's still there we wait 200ms and try again.  It's ugly but it works.

Example failures:
- https://github.com/replicate/cog/actions/runs/11858411614/job/33048999811
- https://github.com/replicate/cog/actions/runs/11858411614/job/33049000193
- and other jobs on the same workflow run

python/tests/server/test_http_input.py

@@ -156,6 +156,15 @@ def test_path_temporary_files_are_removed(client, match):
         },
     )
     temporary_path = resp.json()["output"]
+
+    # HACK: the temp file is deleted in a concurrent.futures callback, which
+    # isn't guaranteed to return before the future result resolves.  Therefore
+    # the file might still exist at the point we receive the HTTP response.
+
+    if not os.path.exists(temporary_path):
+        return  # the file is gone, we're done
+    # else wait and try again
+    time.sleep(0.2)
     assert not os.path.exists(temporary_path)