clean models manager (#9168)

Have only the following Methods:
- `list_models`: getting current available models
- `try_list_models`: sync version no refresh for tui use
- `get_default_model`: get the default model (should be tightened to
core and received on session configuration)
- `get_model_info`: get `ModelInfo` for a specific model (should be
tightened to core but used in tests)
- `refresh_if_new_etag`: trigger refresh on different etags

Also move the cache to its own struct

codex-rs/exec/src/lib.rs

@@ -29,6 +29,7 @@ use codex_core::config::find_codex_home;
 use codex_core::config::load_config_as_toml_with_cli_overrides;
 use codex_core::config::resolve_oss_provider;
 use codex_core::git_info::get_git_repo_root;
+use codex_core::models_manager::manager::RefreshStrategy;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::Event;
 use codex_core::protocol::EventMsg;
@@ -310,7 +311,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     );
     let default_model = thread_manager
         .get_models_manager()
-        .get_model(&config.model, &config)
+        .get_default_model(&config.model, &config, RefreshStrategy::OnlineIfUncached)
         .await;
 
     // Handle resume subcommand by resolving a rollout path and using explicit resume API.