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/app-server/tests/common/models_cache.rs

@@ -36,7 +36,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         default_reasoning_summary: ReasoningSummary::Auto,
         support_verbosity: false,
         default_verbosity: None,
-        availability_nux: None,
+        availability_nux: preset.availability_nux.clone(),
         apply_patch_tool_type: None,
         web_search_tool_type: Default::default(),
         truncation_policy: TruncationPolicyConfig::bytes(/*limit*/ 10_000),