126474: raft: pass logSlice from maybeAppend to append r=pav-kv a=pav-kv

This PR propagates the appended `logSlice` down the stack from `maybeAppend` to `append`. All appends should use this type for readability and safety. Ultimately, the `logSlice` will be passed from the append method to `unstable.truncateAndAppend` which will implement a few more safety checks and use the `logSlice.term` to track the "last accepted term".

This change helped fixing a bunch of bugs in tests that initialized `RawNode` or `raftLog` incorrectly.

Part of #124440

Co-authored-by: Pavel Kalinnikov <[email protected]>