chore: upgrade to Rust 1.89 (#2465)

Codex created this PR from the following prompt:

> upgrade this entire repo to Rust 1.89. Note that this requires
updating codex-rs/rust-toolchain.toml as well as the workflows in
.github/. Make sure that things are "clippy clean" as this change will
likely uncover new Clippy errors. `just fmt` and `cargo clippy --tests`
are sufficient to check for correctness

Note this modifies a lot of lines because it folds nested `if`
statements using `&&`.

---
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* #2467
* __->__ #2465

codex-rs/login/src/server.rs

@@ -364,10 +364,10 @@ async fn persist_tokens_async(
     let codex_home = codex_home.to_path_buf();
     tokio::task::spawn_blocking(move || {
         let auth_file = get_auth_file(&codex_home);
-        if let Some(parent) = auth_file.parent() {
-            if !parent.exists() {
-                std::fs::create_dir_all(parent).map_err(io::Error::other)?;
-            }
+        if let Some(parent) = auth_file.parent()
+            && !parent.exists()
+        {
+            std::fs::create_dir_all(parent).map_err(io::Error::other)?;
         }
 
         let mut auth = read_or_default(&auth_file);