app-server: Add streaming and tty/pty capabilities to command/exec (#13640)

* Add an ability to stream stdin, stdout, and stderr
* Streaming of stdout and stderr has a configurable cap for total amount
of transmitted bytes (with an ability to disable it)
* Add support for overriding environment variables
* Add an ability to terminate running applications (using
`command/exec/terminate`)
* Add TTY/PTY support, with an ability to resize the terminal (using
`command/exec/resize`)

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

@@ -2,6 +2,8 @@ mod account;
 mod analytics;
 mod app_list;
 mod collaboration_mode_list;
+#[cfg(unix)]
+mod command_exec;
 mod compaction;
 mod config_rpc;
 mod connection_handling_websocket;