utils/pty: add streaming spawn and terminal sizing primitives (#13695)

Enhance pty utils:
* Support closing stdin
* Separate stderr and stdout streams to allow consumers differentiate them
* Provide compatibility helper to merge both streams back into combined one
* Support specifying terminal size for pty, including on-demand resizes while process is already running
* Support terminating the process while still consuming its outputs

codex-rs/utils/pty/README.md

@@ -4,22 +4,27 @@ Lightweight helpers for spawning interactive processes either under a PTY (pseud
 
 ## API surface
 
-- `spawn_pty_process(program, args, cwd, env, arg0)` → `SpawnedProcess`
+- `spawn_pty_process(program, args, cwd, env, arg0, size)` → `SpawnedProcess`
 - `spawn_pipe_process(program, args, cwd, env, arg0)` → `SpawnedProcess`
 - `spawn_pipe_process_no_stdin(program, args, cwd, env, arg0)` → `SpawnedProcess`
+- `combine_output_receivers(stdout_rx, stderr_rx)` → `broadcast::Receiver<Vec<u8>>`
 - `conpty_supported()` → `bool` (Windows only; always true elsewhere)
+- `TerminalSize { rows, cols }` selects PTY dimensions in character cells.
 - `ProcessHandle` exposes:
   - `writer_sender()` → `mpsc::Sender<Vec<u8>>` (stdin)
-  - `output_receiver()` → `broadcast::Receiver<Vec<u8>>` (stdout/stderr merged)
+  - `resize(TerminalSize)`
+  - `close_stdin()`
   - `has_exited()`, `exit_code()`, `terminate()`
-- `SpawnedProcess` bundles `handle`, `output_rx`, and `exit_rx` (oneshot exit code).
+- `SpawnedProcess` bundles `session`, `stdout_rx`, `stderr_rx`, and `exit_rx` (oneshot exit code).
 
 ## Usage examples
 
 ```rust
 use std::collections::HashMap;
 use std::path::Path;
+use codex_utils_pty::combine_output_receivers;
 use codex_utils_pty::spawn_pty_process;
+use codex_utils_pty::TerminalSize;
 
 # tokio_test::block_on(async {
 let env_map: HashMap<String, String> = std::env::vars().collect();
@@ -29,13 +34,14 @@ let spawned = spawn_pty_process(
     Path::new("."),
     &env_map,
     &None,
+    TerminalSize::default(),
 ).await?;
 
 let writer = spawned.session.writer_sender();
 writer.send(b"exit\n".to_vec()).await?;
 
 // Collect output until the process exits.
-let mut output_rx = spawned.output_rx;
+let mut output_rx = combine_output_receivers(spawned.stdout_rx, spawned.stderr_rx);
 let mut collected = Vec::new();
 while let Ok(chunk) = output_rx.try_recv() {
     collected.extend_from_slice(&chunk);