Fix stale model test fixtures

[aibrahim-oai]

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.