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/core/src/model_provider_info.rs

@@ -167,10 +167,10 @@ impl ModelProviderInfo {
 
         if let Some(env_headers) = &self.env_http_headers {
             for (header, env_var) in env_headers {
-                if let Ok(val) = std::env::var(env_var) {
-                    if !val.trim().is_empty() {
-                        builder = builder.header(header, val);
-                    }
+                if let Ok(val) = std::env::var(env_var)
+                    && !val.trim().is_empty()
+                {
+                    builder = builder.header(header, val);
                 }
             }
         }