Rename OpenAI models to models manager (#8346)

# 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/codex_delegate.rs

@@ -25,7 +25,7 @@ use crate::codex::Session;
 use crate::codex::TurnContext;
 use crate::config::Config;
 use crate::error::CodexErr;
-use crate::openai_models::models_manager::ModelsManager;
+use crate::models_manager::manager::ModelsManager;
 use codex_protocol::protocol::InitialHistory;
 
 /// Start an interactive sub-Codex conversation and return IO channels.