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/app-server/tests/suite/v2/turn_settings_update.rs

@@ -557,8 +557,8 @@ fn mock_config(codex_home: &Path, server_uri: &str) -> Result<MockResponsesConfi
     let model = bundled_models_response()?
         .models
         .into_iter()
-        .find(|model| model.slug == "gpt-5.4")
-        .context("bundled catalog should include gpt-5.4")?;
+        .find(|model| model.slug == "gpt-5.5")
+        .context("bundled catalog should include gpt-5.5")?;
     let models = [MODEL_A, MODEL_B]
         .into_iter()
         .map(|slug| {

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -4063,7 +4063,7 @@ async fn turn_start_emits_spawn_agent_item_with_model_metadata_v2() -> Result<()
     const PARENT_PROMPT: &str = "spawn a child and continue";
     const SPAWN_CALL_ID: &str = "spawn-call-1";
     const CHILD_PLAN_CALL_ID: &str = "child-plan-call";
-    const REQUESTED_MODEL: &str = "gpt-5.2";
+    const REQUESTED_MODEL: &str = "gpt-5.5";
     const REQUESTED_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::Low;
 
     let server = responses::start_mock_server().await;
@@ -4719,7 +4719,7 @@ async fn turn_start_emits_spawn_agent_item_with_effective_role_model_metadata_v2
     const CHILD_PROMPT: &str = "child: do work";
     const PARENT_PROMPT: &str = "spawn a child and continue";
     const SPAWN_CALL_ID: &str = "spawn-call-1";
-    const REQUESTED_MODEL: &str = "gpt-5.2";
+    const REQUESTED_MODEL: &str = "gpt-5.5";
     const REQUESTED_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::Low;
     const ROLE_MODEL: &str = "gpt-5.4";
     const ROLE_REASONING_EFFORT: ReasoningEffort = ReasoningEffort::High;