Connection issue when backlog quota is exceeded - unable to consume

It seems that there's an edge case that's not handled properly when a backlog quota is exceeded.
The clients aren't able to connect, and I understand that they shouldn't be able to produce, but consumption gets blocked as well, even if the downstream topic has no subscribers (so the messages should be easily consumed, thus resolving the backlog.)

Start by reproducing the case where a backlog quota is exceeded and clients cannot consume, especially when the downstream topic has no subscribers. Trace the quota and connection handling involved in that scenario. Done means consumption remains possible while production is blocked, allowing the backlog to drain.

Written by the indexing model from the issue text.