Add thread/shellCommand to app server API surface (#14988)

This PR adds a new `thread/shellCommand` app server API so clients can
implement `!` shell commands. These commands are executed within the
sandbox, and the command text and output are visible to the model.

The internal implementation mirrors the current TUI `!` behavior.
- persist shell command execution as `CommandExecution` thread items,
including source and formatted output metadata
- bridge live and replayed app-server command execution events back into
the existing `tui_app_server` exec rendering path

This PR also wires `tui_app_server` to submit `!` commands through the
new API.

codex-rs/core/src/tasks/user_shell.rs

@@ -331,6 +331,9 @@ async fn persist_user_shell_output(
         session
             .record_conversation_items(turn_context, std::slice::from_ref(&output_item))
             .await;
+        // Standalone shell turns can run before any regular user turn, so
+        // explicitly materialize rollout persistence after recording output.
+        session.ensure_rollout_materialized().await;
         return;
     }