Remove retired model entries while preserving migration prompts (#44250)

## Why

Saved model selections can outlive their bundled catalog entries and still need migration guidance.

## What changed

- Remove `gpt-5.2` and `gpt-5.4-mini` from the bundled model catalog.
- Preserve migration from saved `gpt-5.4-mini` selections to `gpt-5.6-luna` when the old model is absent, while respecting catalog-provided migration metadata.
- Update tests and snapshots to use current models or explicit test fixtures, and remove the obsolete prompt fixture.

## Testing

Extend migration tests to cover catalog metadata precedence, saved selections, replacement model and reasoning-effort persistence, acknowledged prompts, and missing or hidden migration targets.

GitOrigin-RevId: 82beea504cc4a15699677c28a61ec0939af3069c

codex-rs/core/tests/suite/compact.rs

@@ -407,9 +407,12 @@ fn model_info_with_context_window(slug: &str, context_window: i64) -> ModelInfo
     let mut model_info = models_response
         .models
         .into_iter()
-        .find(|model| model.slug == slug)
+        .find(|model| model.slug == "gpt-5.5")
         .expect("model missing from models.json");
+    model_info.slug = slug.to_string();
     model_info.context_window = Some(context_window);
+    model_info.max_context_window = Some(context_window);
+    model_info.comp_hash = None;
     model_info
 }