Client hangs while holding open a streaming upload connection

[mullr]

1. Start to stream an upload
2. While the upload connection is still open, try to download some objects

Step 2 hangs at least intermittently.

[mullr]

I have a test case and fix for this up at https://github.com/mullr/fake-gcs-server/tree/upload-deadlock. But they are AI generated, and I don't want to impose that on the maintainer here. Perhaps the test case could be useful, for developing a better fix.

[mullr]

I tested this against #2117, but that didn't fix this problem.