Show migration link (#8228)

# 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/protocol/src/openai_models.rs

@@ -52,6 +52,7 @@ pub struct ModelUpgrade {
     pub id: String,
     pub reasoning_effort_mapping: Option<HashMap<ReasoningEffort, ReasoningEffort>>,
     pub migration_config_key: String,
+    pub model_link: Option<String>,
 }
 
 /// Metadata describing a Codex-supported model.
@@ -216,6 +217,8 @@ impl From<ModelInfo> for ModelPreset {
                     &info.supported_reasoning_levels,
                 ),
                 migration_config_key: info.slug.clone(),
+                // todo(aibrahim): add the model link here.
+                model_link: None,
             }),
             show_in_picker: info.visibility == ModelVisibility::List,
             supported_in_api: info.supported_in_api,