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/execpolicy/src/sed_command.rs

@@ -3,12 +3,12 @@ use crate::error::Result;
 
 pub fn parse_sed_command(sed_command: &str) -> Result<()> {
     // For now, we parse only commands like `122,202p`.
-    if let Some(stripped) = sed_command.strip_suffix("p") {
-        if let Some((first, rest)) = stripped.split_once(",") {
-            if first.parse::<u64>().is_ok() && rest.parse::<u64>().is_ok() {
-                return Ok(());
-            }
-        }
+    if let Some(stripped) = sed_command.strip_suffix("p")
+        && let Some((first, rest)) = stripped.split_once(",")
+        && first.parse::<u64>().is_ok()
+        && rest.parse::<u64>().is_ok()
+    {
+        return Ok(());
     }
 
     Err(Error::SedCommandNotProvablySafe {