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/core/tests/suite/remote_models.rs

@@ -381,12 +381,11 @@ async fn remote_models_hide_picker_only_models() -> Result<()> {
     assert_eq!(selected, "gpt-5.2-codex");
 
     let available = manager.list_models(&config).await;
-    assert!(
-        available
-            .iter()
-            .all(|model| model.model != "codex-auto-balanced"),
-        "hidden models should not appear in the picker list"
-    );
+    let hidden = available
+        .iter()
+        .find(|model| model.model == "codex-auto-balanced")
+        .expect("hidden remote model should be listed");
+    assert!(!hidden.show_in_picker, "hidden models should remain hidden");
 
     Ok(())
 }