fix(core) model_info preserves slug (#11602)

## Summary
Preserve the specified model slug when we get a prefix-based match

## Testing
- [x] added unit test

---------

Co-authored-by: Ahmed Ibrahim <aibrahim@openai.com>

codex-rs/core/src/models_manager/manager.rs

@@ -141,7 +141,10 @@ impl ModelsManager {
             .find_remote_model_by_longest_prefix(model, config)
             .await;
         let model = if let Some(remote) = remote {
-            remote
+            ModelInfo {
+                slug: model.to_string(),
+                ..remote
+            }
         } else {
             model_info::model_info_from_slug(model)
         };