Report app-server version in codex doctor (#24311)

## Why

We are seeing cases where users have an old background app-server still
running. `codex doctor` already reports background server state, but
without the running app-server version it is harder to diagnose
behaviors that depend on the daemon build.

## What changed

- Reused the app-server daemon's passive initialize probe through a
narrow `probe_app_server_version` helper.
- Updated the `codex doctor` Background Server section to report
`app-server version: <version>` when the socket is reachable.
- Preserved the not-running OK behavior and report `app-server version:
unavailable (<short error>)` when a socket exists but the passive probe
fails.

codex-rs/app-server-daemon/src/lib.rs

@@ -74,6 +74,12 @@ pub struct BootstrapOptions {
     pub remote_control_enabled: bool,
 }
 
+/// Passively probes an existing app-server socket and returns its reported
+/// app-server version.
+pub async fn probe_app_server_version(socket_path: &Path) -> Result<String> {
+    Ok(client::probe(socket_path).await?.app_server_version)
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
 #[serde(rename_all = "camelCase")]
 pub enum BootstrapStatus {