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)
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* #2467
* __->__ #2465

.github/workflows/rust-ci.yml

@@ -57,7 +57,7 @@ jobs:
         working-directory: codex-rs
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.88
+      - uses: dtolnay/rust-toolchain@1.89
         with:
           components: rustfmt
       - name: cargo fmt
@@ -112,7 +112,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@1.88
+      - uses: dtolnay/rust-toolchain@1.89
         with:
           targets: ${{ matrix.target }}
           components: clippy