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/chatwidget/snapshots/codex_tui__chatwidget__tests__model_picker_filters_hidden_models.snap

@@ -0,0 +1,11 @@
+---
+source: tui/src/chatwidget/tests.rs
+assertion_line: 1989
+expression: popup
+---
+  Select Model and Effort
+  Access legacy models by running codex -m <model_name> or in your config.toml
+
+› 1. test-visible-model (current)  test-visible-model description
+
+  Press enter to select reasoning effort, or esc to dismiss.