feat: update default (#4076)

Changes:
- Default model and docs now use gpt-5-codex. 
- Disables the GPT-5 Codex NUX by default.
- Keeps presets available for API key users.

codex-rs/protocol/src/mcp_protocol.rs

@@ -710,7 +710,7 @@ mod tests {
         let request = ClientRequest::NewConversation {
             request_id: RequestId::Integer(42),
             params: NewConversationParams {
-                model: Some("gpt-5".to_string()),
+                model: Some("gpt-5-codex".to_string()),
                 profile: None,
                 cwd: None,
                 approval_policy: Some(AskForApproval::OnRequest),
@@ -726,7 +726,7 @@ mod tests {
                 "method": "newConversation",
                 "id": 42,
                 "params": {
-                    "model": "gpt-5",
+                    "model": "gpt-5-codex",
                     "approvalPolicy": "on-request"
                 }
             }),