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/exec/src/event_processor.rs

@@ -29,9 +29,9 @@ pub(crate) fn handle_last_message(last_agent_message: Option<&str>, output_file:
 }
 
 fn write_last_message_file(contents: &str, last_message_path: Option<&Path>) {
-    if let Some(path) = last_message_path {
-        if let Err(e) = std::fs::write(path, contents) {
-            eprintln!("Failed to write last message file {path:?}: {e}");
-        }
+    if let Some(path) = last_message_path
+        && let Err(e) = std::fs::write(path, contents)
+    {
+        eprintln!("Failed to write last message file {path:?}: {e}");
     }
 }