We need to exhaustively test the invariants for the engine states in separate raft logs.
We'll add assertions but should also explore these using datadriven tests.

Some of the code in #88606 might be helpful.

TODO:

• crash in the middle of snapshot ingestion
• crash in the middle of replica subsumption (snapshot and merge trigger case)
• crash in the middle of replica creation
• crash in the middle of split
• crash in the middle of merge
• crash in the middle of replica destruction
• general engine-level testing of all of the above.

I'll add that our testing of the snapshot SSTs is currently weak. We rely on spinning up a real cluster and then intercept the SSTs, which is not very maintainable. We should complement this with kvstorage-level tests that create a snapshot and examine in detail their contents and the engine each one targets.

Epic CRDB-55218