set gpt-5 as default model for Windows users (#4676)

Codex isn’t great yet on Windows outside of WSL, and while we’ve merged
https://github.com/openai/codex/pull/4269 to reduce the repetitive
manual approvals on readonly commands, we’ve noticed that users seem to
have more issues with GPT-5-Codex than with GPT-5 on Windows.

This change makes GPT-5 the default for Windows users while we continue
to improve the CLI harness and model for GPT-5-Codex on Windows.

codex-rs/tui/src/chatwidget/tests.rs

@@ -8,6 +8,7 @@ use codex_core::CodexAuth;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config::ConfigToml;
+use codex_core::config::OPENAI_DEFAULT_MODEL;
 use codex_core::protocol::AgentMessageDeltaEvent;
 use codex_core::protocol::AgentMessageEvent;
 use codex_core::protocol::AgentReasoningDeltaEvent;
@@ -1101,6 +1102,11 @@ fn disabled_slash_command_while_task_running_snapshot() {
 
 #[tokio::test]
 async fn binary_size_transcript_snapshot() {
+    // the snapshot in this test depends on gpt-5-codex. Skip for now. We will consider
+    // creating snapshots for other models in the future.
+    if OPENAI_DEFAULT_MODEL != "gpt-5-codex" {
+        return;
+    }
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual();
 
     // Set up a VT100 test terminal to capture ANSI visual output