Pending requests should be resolved when heartbeat eviction removes a dead node

[GenFlogi]

Description

When a server node dies, fetchSockets() (and serverSideEmitWithAck()) requests that are
already in-flight will always wait the full requestsTimeout before failing — even if the
dead node is evicted from nodesMap during the wait due to heartbeat timeout.

This happens because expectedResponseCount is computed once at the start of
fetchSockets() via serverCount() and never updated:

socket.io-mongo-adapter/lib/index.ts

Line 686 in eff10ab

const expectedResponseCount = (await this.serverCount()) - 1;

[GenFlogi]

I noticed this happening in our project since we have a similar problem as in #16

I will post a PR in a minute that attempts to solve this issue. I am not 100% sure if the way I fixed this is a good idea in the context of the whole project.

I will also admit that part of the code has been written with Claude