fix: typos in model picker (#6859)

# External (non-OpenAI) Pull Request Requirements

Before opening this Pull Request, please read the dedicated
"Contributing" markdown file or your PR may be closed:
https://github.com/openai/codex/blob/main/docs/contributing.md

If your PR conforms to our contribution guidelines, replace this text
with a detailed and high quality description of your changes.

Include a link to a bug report or enhancement request.

codex-rs/core/src/client_common.rs

@@ -431,7 +431,7 @@ mod tests {
                 expects_apply_patch_instructions: false,
             },
             InstructionsTestCase {
-                slug: "gpt-5.1-codex",
+                slug: "arcticfox",
                 expects_apply_patch_instructions: false,
             },
         ];