Remove legacy app-server notification handling from tui_app_server (#15390)

As part of moving the TUI onto the app server, we added some temporary
handling of some legacy events. We've confirmed that these do not need
to be supported, so this PR removes this support from the
tui_app_server, allowing for additional simplifications in follow-on
PRs. These events are needed only for very old rollouts. None of the
other app server clients (IDE extension or app) support these either.

## Summary
- stop translating legacy `codex/event/*` notifications inside
`tui_app_server`
- remove the TUI-side legacy warning and rollback buffering/replay paths
that were only fed by those notifications
- keep the lower-level app-server and app-server-client legacy event
plumbing intact so PR #15106 can rebase on top and handle the remaining
exec/lower-layer migration separately

Author: etraut-openai