Update models.json and related fixtures (#19323)

Supersedes #18735.

The scheduled rust-release-prepare workflow force-pushed
`bot/update-models-json` back to the generated models.json-only diff,
which dropped the test and snapshot updates needed for CI.

This PR keeps the latest generated `models.json` from #18735 and adds
the corresponding fixture updates:
- preserve model availability NUX in the app-server model cache fixture
- update core/TUI expectations for the new `gpt-5.4` `xhigh` default
reasoning
- refresh affected TUI chatwidget snapshots for the `gpt-5.5`
default/model copy changes

Validation run locally while preparing the fix:
- `just fmt`
- `cargo test -p codex-app-server model_list`
- `cargo test -p codex-core includes_no_effort_in_request`
- `cargo test -p codex-core
includes_default_reasoning_effort_in_request_when_defined_by_model_info`
- `cargo test -p codex-tui --lib chatwidget::tests`
- `cargo insta pending-snapshots`

---------

Co-authored-by: aibrahim-oai <219906144+aibrahim-oai@users.noreply.github.com>

codex-rs/tui/src/chatwidget/tests/plan_mode.rs

@@ -205,8 +205,7 @@ async fn reasoning_selection_in_plan_mode_without_effort_change_does_not_open_sc
     let _ = drain_insert_history(&mut rx);
     set_chatgpt_auth(&mut chat);
 
-    let current_preset = get_available_model(&chat, "gpt-5.4");
-    chat.set_reasoning_effort(Some(current_preset.default_reasoning_effort));
+    chat.set_reasoning_effort(Some(ReasoningEffortConfig::Medium));
 
     let preset = get_available_model(&chat, "gpt-5.4");
     chat.open_reasoning_popup(preset);

codex-rs/tui/src/chatwidget/tests/status_command_tests.rs

@@ -90,7 +90,7 @@ async fn status_command_uses_catalog_default_reasoning_when_config_empty() {
         other => panic!("expected status output, got {other:?}"),
     };
     assert!(
-        rendered.contains("gpt-5.4 (reasoning medium, summaries auto)"),
+        rendered.contains("gpt-5.4 (reasoning xhigh, summaries auto)"),
         "expected /status to render the catalog default reasoning effort, got: {rendered}"
     );
 }