Revert tui code so it does not rely on in-process app server (#14899)

PR https://github.com/openai/codex/pull/14512 added an in-process app
server and started to wire up the tui to use it. We were originally
planning to modify the `tui` code in place, converting it to use the app
server a bit at a time using a hybrid adapter. We've since decided to
create an entirely new parallel `tui_app_server` implementation and do
the conversion all at once but retain the existing `tui` while we work
the bugs out of the new implementation.

This PR undoes the changes to the `tui` made in the PR #14512 and
restores the old initialization to its previous state. This allows us to
modify the `tui_app_server` without the risk of regressing the old `tui`
code. For example, we can start to remove support for all legacy core
events, like the ones that PR https://github.com/openai/codex/pull/14892
needed to ignore.

Testing:
* I manually verified that the old `tui` starts and shuts down without a
problem.

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

@@ -1,72 +0,0 @@
-/*
-This module holds the temporary adapter layer between the TUI and the app
-server during the hybrid migration period.
-
-For now, the TUI still owns its existing direct-core behavior, but startup
-allocates a local in-process app server and drains its event stream. Keeping
-the app-server-specific wiring here keeps that transitional logic out of the
-main `app.rs` orchestration path.
-
-As more TUI flows move onto the app-server surface directly, this adapter
-should shrink and eventually disappear.
-*/
-
-use super::App;
-use codex_app_server_client::InProcessAppServerClient;
-use codex_app_server_client::InProcessServerEvent;
-use codex_app_server_protocol::JSONRPCErrorError;
-
-impl App {
-    pub(super) async fn handle_app_server_event(
-        &mut self,
-        app_server_client: &InProcessAppServerClient,
-        event: InProcessServerEvent,
-    ) {
-        match event {
-            InProcessServerEvent::Lagged { skipped } => {
-                tracing::warn!(
-                    skipped,
-                    "app-server event consumer lagged; dropping ignored events"
-                );
-            }
-            InProcessServerEvent::ServerNotification(_) => {}
-            InProcessServerEvent::LegacyNotification(_) => {}
-            InProcessServerEvent::ServerRequest(request) => {
-                let request_id = request.id().clone();
-                tracing::warn!(
-                    ?request_id,
-                    "rejecting app-server request while TUI still uses direct core APIs"
-                );
-                if let Err(err) = self
-                    .reject_app_server_request(
-                        app_server_client,
-                        request_id,
-                        "TUI client does not yet handle this app-server server request".to_string(),
-                    )
-                    .await
-                {
-                    tracing::warn!("{err}");
-                }
-            }
-        }
-    }
-
-    async fn reject_app_server_request(
-        &self,
-        app_server_client: &InProcessAppServerClient,
-        request_id: codex_app_server_protocol::RequestId,
-        reason: String,
-    ) -> std::result::Result<(), String> {
-        app_server_client
-            .reject_server_request(
-                request_id,
-                JSONRPCErrorError {
-                    code: -32000,
-                    message: reason,
-                    data: None,
-                },
-            )
-            .await
-            .map_err(|err| format!("failed to reject app-server request: {err}"))
-    }
-}