Refresh on models etag mismatch (#8491)

- Send models etag
- Refresh models on 412
- This wires `ModelsManager` to `ModelFamily` so we don't mutate it
mid-turn

codex-rs/codex-api/src/common.rs

@@ -59,6 +59,7 @@ pub enum ResponseEvent {
         summary_index: i64,
     },
     RateLimits(RateLimitSnapshot),
+    ModelsEtag(String),
 }
 
 #[derive(Debug, Serialize, Clone)]