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

codex-rs/mcp-server/src/codex_message_processor.rs

@@ -154,7 +154,7 @@ impl CodexMessageProcessor {
             Error(JSONRPCErrorError),
         }
 
-        let reply = match run_login_server(opts, None) {
+        let reply = match run_login_server(opts) {
             Ok(server) => {
                 let login_id = Uuid::new_v4();
                 let shutdown_handle = server.cancel_handle();