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/app-server/tests/suite/v2/mod.rs

@@ -38,6 +38,7 @@ mod thread_name_websocket;
 mod thread_read;
 mod thread_resume;
 mod thread_rollback;
+mod thread_shell_command;
 mod thread_start;
 mod thread_status;
 mod thread_unarchive;