fix: remove shutdown_flag param to run_login_server() (#2399)

In practice, this was always passed in as `None`, so eliminated the
param and updated all the call sites.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2399).
* __->__ #2399
* #2398
* #2396
* #2395
* #2394
* #2393
* #2389

codex-rs/login/tests/login_server_e2e.rs

@@ -101,7 +101,7 @@ async fn end_to_end_login_flow_persists_auth_json() {
         open_browser: false,
         force_state: Some(state),
     };
-    let server = run_login_server(opts, None).unwrap();
+    let server = run_login_server(opts).unwrap();
     let login_port = server.actual_port;
 
     // Simulate browser callback, and follow redirect to /success
@@ -159,7 +159,7 @@ async fn creates_missing_codex_home_dir() {
         open_browser: false,
         force_state: Some(state),
     };
-    let server = run_login_server(opts, None).unwrap();
+    let server = run_login_server(opts).unwrap();
     let login_port = server.actual_port;
 
     let client = reqwest::Client::new();