Finish moving codex exec to app-server (#15424)

This PR completes the conversion of non-interactive `codex exec` to use
app server rather than directly using core events and methods.

### Summary
- move `codex-exec` off exec-owned `AuthManager` and `ThreadManager`
state
- route exec bootstrap, resume, and auth refresh through existing
app-server paths
- replace legacy `codex/event/*` decoding in exec with typed app-server
notification handling
- update human and JSONL exec output adapters to translate existing
app-server notifications only
- clean up "app server client" layer by eliminating support for legacy
notifications; this is no longer needed
- remove exposure of `authManager` and `threadManager` from "app server
client" layer

### Testing
- `exec` has pretty extensive unit and integration tests already, and
these all pass
- In addition, I asked Codex to put together a comprehensive manual set
of tests to cover all of the `codex exec` functionality (including
command-line options), and it successfully generated and ran these tests

codex-rs/tui_app_server/src/app/app_server_adapter.rs

@@ -122,9 +122,6 @@ impl App {
                 self.handle_server_notification_event(app_server_client, notification)
                     .await;
             }
-            AppServerEvent::LegacyNotification(_) => {
-                tracing::debug!("ignoring legacy app-server notification in tui_app_server");
-            }
             AppServerEvent::ServerRequest(request) => {
                 self.handle_server_request_event(app_server_client, request)
                     .await;

codex-rs/tui_app_server/src/onboarding/onboarding_screen.rs

@@ -504,7 +504,6 @@ pub(crate) async fn run_onboarding_app(
                             return Err(color_eyre::eyre::eyre!(message));
                         }
                         AppServerEvent::Lagged { .. }
-                        | AppServerEvent::LegacyNotification(_)
                         | AppServerEvent::ServerRequest(_) => {}
                     }
                 }