Enable model upgrade popup even when selected model is no longer in picker (#8802)

With `config.toml`:
```
model = "gpt-5.1-codex"
```
(where `gpt-5.1-codex` has `show_in_picker: false` in
[`model_presets.rs`](https://github.com/openai/codex/blob/main/codex-rs/core/src/models_manager/model_presets.rs);
this happens if the user hasn't used codex in a while so they didn't see
the popup before their model was changed to `show_in_picker: false`)

The upgrade picker used to not show (because `gpt-5.1-codex` was
filtered out of the model list in code). Now, the filtering is done
downstream in tui and app-server, so the model upgrade popup shows:

<img width="1503" height="227" alt="Screenshot 2026-01-06 at 5 04 37 PM"
src="https://github.com/user-attachments/assets/26144cc2-0b3f-4674-ac17-e476781ec548"
/>

codex-rs/tui/src/app.rs

@@ -1334,16 +1334,19 @@ mod tests {
     use codex_core::AuthManager;
     use codex_core::CodexAuth;
     use codex_core::ConversationManager;
+    use codex_core::config::ConfigBuilder;
     use codex_core::protocol::AskForApproval;
     use codex_core::protocol::Event;
     use codex_core::protocol::EventMsg;
     use codex_core::protocol::SandboxPolicy;
     use codex_core::protocol::SessionConfiguredEvent;
     use codex_protocol::ConversationId;
+    use insta::assert_snapshot;
     use ratatui::prelude::Line;
     use std::path::PathBuf;
     use std::sync::Arc;
     use std::sync::atomic::AtomicBool;
+    use tempfile::tempdir;
 
     async fn make_test_app() -> App {
         let (chat_widget, app_event_tx, _rx, _op_rx) = make_chatwidget_manual_with_sender().await;
@@ -1427,6 +1430,24 @@ mod tests {
         codex_core::models_manager::model_presets::all_model_presets().clone()
     }
 
+    fn model_migration_copy_to_plain_text(
+        copy: &crate::model_migration::ModelMigrationCopy,
+    ) -> String {
+        let mut s = String::new();
+        for span in &copy.heading {
+            s.push_str(&span.content);
+        }
+        s.push('\n');
+        s.push('\n');
+        for line in &copy.content {
+            for span in &line.spans {
+                s.push_str(&span.content);
+            }
+            s.push('\n');
+        }
+        s
+    }
+
     #[tokio::test]
     async fn model_migration_prompt_only_shows_for_deprecated_models() {
         let seen = BTreeMap::new();
@@ -1508,6 +1529,59 @@ mod tests {
         assert!(target_preset_for_upgrade(&available, "missing-target").is_none());
     }
 
+    #[tokio::test]
+    async fn model_migration_prompt_shows_for_hidden_model() {
+        let codex_home = tempdir().expect("temp codex home");
+        let config = ConfigBuilder::default()
+            .codex_home(codex_home.path().to_path_buf())
+            .build()
+            .await
+            .expect("config");
+
+        let available_models = all_model_presets();
+        let current = available_models
+            .iter()
+            .find(|preset| preset.model == "gpt-5.1-codex")
+            .cloned()
+            .expect("gpt-5.1-codex preset present");
+        assert!(
+            !current.show_in_picker,
+            "expected gpt-5.1-codex to be hidden from picker for this test"
+        );
+
+        let upgrade = current.upgrade.as_ref().expect("upgrade configured");
+        assert!(
+            should_show_model_migration_prompt(
+                &current.model,
+                &upgrade.id,
+                &config.notices.model_migrations,
+                &available_models,
+            ),
+            "expected migration prompt to be eligible for hidden model"
+        );
+
+        let target = target_preset_for_upgrade(&available_models, &upgrade.id)
+            .expect("upgrade target present");
+        let target_description =
+            (!target.description.is_empty()).then(|| target.description.clone());
+        let can_opt_out = true;
+        let copy = migration_copy_for_models(
+            &current.model,
+            &upgrade.id,
+            upgrade.model_link.clone(),
+            upgrade.upgrade_copy.clone(),
+            target.display_name.clone(),
+            target_description,
+            can_opt_out,
+        );
+
+        // Snapshot the copy we would show; rendering is covered by model_migration snapshots.
+        assert_snapshot!(
+            "model_migration_prompt_shows_for_hidden_model",
+            model_migration_copy_to_plain_text(&copy)
+        );
+    }
+
     #[tokio::test]
     async fn update_reasoning_effort_updates_config() {
         let mut app = make_test_app().await;