fix: eliminate ServerOptions.login_timeout and have caller use tokio::time::timeout() instead (#2395)

https://github.com/openai/codex/pull/2373 introduced
`ServerOptions.login_timeout` and `spawn_timeout_watcher()` to use an
extra thread to manage the timeout for the login server. Now that we
have asyncified the login stack, we can use `tokio::time::timeout()`
from "outside" the login library to manage the timeout rather than
having to a commit to a specific "timeout" concept from within.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2395).
* #2399
* #2398
* #2396
* __->__ #2395
* #2394
* #2393
* #2389

codex-rs/login/tests/login_server_e2e.rs

@@ -100,7 +100,6 @@ async fn end_to_end_login_flow_persists_auth_json() {
         port: 0,
         open_browser: false,
         force_state: Some(state),
-        login_timeout: None,
     };
     let server = run_login_server(opts, None).unwrap();
     let login_port = server.actual_port;
@@ -159,7 +158,6 @@ async fn creates_missing_codex_home_dir() {
         port: 0,
         open_browser: false,
         force_state: Some(state),
-        login_timeout: None,
     };
     let server = run_login_server(opts, None).unwrap();
     let login_port = server.actual_port;