Fix stale model test fixtures (#18719)

Fixes stale test fixtures left after the active bundled model catalog
updates in #18586 and #18388. Those changes made `gpt-5.4` the current
default and removed several older hardcoded slugs, which left Windows
Bazel shards failing TUI and config tests.

What changed:
- Refresh TUI model migration, availability NUX, plan-mode, status, and
snapshot fixtures to use active bundled model slugs.
- Update the config edit test expectation for the TOML-quoted
`"gpt-5.2"` migration key.
- Move the model catalog tests into
`codex-rs/tui/src/app/tests/model_catalog.rs` so touching them does not
trip the blob-size policy for `app.rs`.

Verification:
- CI Bazel/lint checks are expected to cover the affected test shards.

codex-rs/tui/src/snapshots/codex_tui__app__tests__model_migration_prompt_shows_for_hidden_model.snap

@@ -1,11 +0,0 @@
----
-source: tui/src/app.rs
-expression: model_migration_copy_to_plain_text(&copy)
----
-**Codex just got an upgrade. Introducing gpt-5.3-codex.**
-
-Codex is now powered by gpt-5.3-codex, our most capable agentic coding model yet. It's built for long-running, project-scale work, with mid-turn steering + frequent progress updates so you can collaborate while it runs (and it's faster too).
-
-Learn more: https://openai.com/index/introducing-gpt-5-3-codex/
-
-You can keep using gpt-5.1-codex if you prefer.