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/core/tests/suite/prompt_caching.rs

@@ -85,7 +85,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
         .await?;
     let base_instructions = thread_manager
         .get_models_manager()
-        .construct_model_info(
+        .get_model_info(
             config
                 .model
                 .as_deref()