fix(ads): check if channel is closed

[lukidzi]

We have observed that sometimes writing to the channel happens when the channel is already closed. This issue is related to #1023.

Upon reviewing the code, I found that the value channel can be created by one goroutine, while a different goroutine handles sending data on this channel. The first goroutine depends on the existence of the stream and requests, while the second one is triggered from SetSnapshot, which operates with a different context than the server.

There appears to be a race condition where the stream is canceled, and resources start to be cleaned up, while the context in SetSnapshot might cancel later, leading to writing on a closed channel.

This is not a complete fix but should prevent the panic. We should aim to fix this by possibly keeping the context within the watch and checking for its cancellation, or other?