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 `&&`.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2465).
* #2467
* __->__ #2465

codex-rs/tui/tests/vt100_history.rs

@@ -112,12 +112,11 @@ fn long_token_wraps() {
     let mut count_a = 0usize;
     for row in 0..6 {
         for col in 0..20 {
-            if let Some(cell) = screen.cell(row, col) {
-                if let Some(ch) = cell.contents().chars().next() {
-                    if ch == 'A' {
-                        count_a += 1;
-                    }
-                }
+            if let Some(cell) = screen.cell(row, col)
+                && let Some(ch) = cell.contents().chars().next()
+                && ch == 'A'
+            {
+                count_a += 1;
             }
         }
     }