Remove core protocol dependency [1/2] (#20324)

## Why

This stack moves `codex-tui` away from the core protocol event surface
and toward app-server API shapes plus TUI-owned local models. This first
PR sets up the lower-risk foundation: it introduces the local model
surface and extracts app-server event routing into focused TUI modules
while preserving the existing behavior for the larger migration in PR2.

This PR is part 1 of a 2-PR stack:

1. Add TUI-owned replacement models and extract app-server event
routing.
2. Move the active TUI flow to app-server notifications and delete
obsolete adapter code.

## What changed

- Added TUI-owned approval, diff, session state, session resume, token
usage, and user-message models.
- Added `app/app_server_event_targets.rs` and `app/app_server_events.rs`
to hold app-server event targeting and dispatch logic outside `app.rs`.
- Updated app/status tests to use the local model layer and added
focused routing coverage.
- Boxed a few large async TUI test futures so this base layer remains
checkable without overflowing the default test stack.

## Verification

- `cargo check -p codex-tui --tests`

codex-rs/tui/src/app.rs

@@ -183,7 +183,8 @@ use tokio::task::JoinHandle;
 use toml::Value as TomlValue;
 use uuid::Uuid;
 mod agent_navigation;
-mod app_server_adapter;
+mod app_server_event_targets;
+mod app_server_events;
 pub(crate) mod app_server_requests;
 mod background_requests;
 mod config_persistence;