TAIL and SINK can miss writes due to conccurrent DDL

[jkosh44]

What version of Materialize are you using?

main

How did you install Materialize?

Built from source

What is the issue?

Group commit causes writes to be asynchronous. If a table is deleted while a write is pending in group commit, then we just respond to the client with an OK. See below for more details: https://github.com/MaterializeInc/materialize/blob/19c9736241017477be9d996d45d44e6a7b8a8e07/src/adapter/src/coord/appends.rs#L243-L250

The issue is that if someone is tailing a table when this happens, then they will never see the write even though we responded with an OK for the write.

DDL was recently updated to explicitly trigger a group commit. That means that before deleting a table, we drain all pending writes. Therefore, it's impossible for a table to be deleted before a the write is sent. So we can remove this check and instead replace it with an assert that the collection exists.

Relevant log output

No response