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/cli/src/doctor.rs

@@ -392,11 +392,11 @@ async fn build_report(
                     })
                 },
                 run_async_check("state", progress.clone(), state_check(config)),
-                async {
-                    run_sync_check("app-server", progress.clone(), || {
-                        background_server_check(config)
-                    })
-                },
+                run_async_check(
+                    "app-server",
+                    progress.clone(),
+                    background_server_check(config)
+                ),
                 run_async_check(
                     "provider reachability",
                     progress.clone(),