Add workspace messages app-server API (#29001)
## Summary
- Add backend-client types and fetch support for active workspace
messages.
- Add the app-server v2 `account/workspaceMessages/read` method,
generated schemas, and README documentation.
- Delegate workspace-message eligibility to the Codex backend feature
gate; map a backend 404 to `featureEnabled: false`.
## Testing
- `just write-app-server-schema`
- `just test -p codex-backend-client`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server workspace_messages`
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server`
- `just fmt`
## Stack
- Base PR for #28232, which adds the TUI status-line integration.
xli-oai