fix: use brew upgrade --cask codex to avoid warnings and ambiguity (#9823)

Fixes #9822 

### Summary

Make the Homebrew upgrade command explicit by using `brew upgrade --cask
codex`.

### Motivation

During the Codex self-update, Homebrew can emit an avoidable warning
because the
name `codex` resolves to a cask:

```
Warning: Formula codex was renamed to homebrew/cask/codex.
````

While the upgrade succeeds, this relies on implicit name resolution and
produces
unnecessary output during the update flow.

### Why `--cask`

* Eliminates warning/noise for users
* Explicitly matches how Codex is distributed via Homebrew
* Avoids reliance on name resolution behavior
* Makes the command more robust if a `codex` formula is ever introduced

### Context

This restores the `--cask` flag that was removed in #6238 after being
considered
“not necessary” during review:
[https://github.com/openai/codex/pull/6238#discussion_r2505947880](https://github.com/openai/codex/pull/6238#discussion_r2505947880).

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/tui/src/update_action.rs

@@ -15,7 +15,7 @@ impl UpdateAction {
         match self {
             UpdateAction::NpmGlobalLatest => ("npm", &["install", "-g", "@openai/codex"]),
             UpdateAction::BunGlobalLatest => ("bun", &["install", "-g", "@openai/codex"]),
-            UpdateAction::BrewUpgrade => ("brew", &["upgrade", "codex"]),
+            UpdateAction::BrewUpgrade => ("brew", &["upgrade", "--cask", "codex"]),
         }
     }