Persist update dismissal without cache (#27783)

## Summary

Choosing “Don’t remind me” can silently fail when `version.json`
disappears before dismissal because `dismiss_version` returns success
without writing anything. The same update can then reappear on the next
launch.

Initialize a minimal `VersionInfo` from the selected version when the
cache cannot be read, then persist the dismissal through the existing
write path.

Fixes #27147

codex-rs/tui/src/updates_cache_tests.rs

@@ -0,0 +1,29 @@
+use super::*;
+use crate::legacy_core::config::ConfigBuilder;
+use pretty_assertions::assert_eq;
+use tempfile::tempdir;
+
+#[tokio::test]
+async fn dismiss_version_creates_cache_file_when_missing() {
+    let codex_home = tempdir().expect("temp codex home");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .build()
+        .await
+        .expect("load config");
+    let version_file = version_filepath(&config);
+
+    dismiss_version(&config, "999.0.0")
+        .await
+        .expect("dismiss version");
+
+    let info = read_version_info(&version_file).expect("read version info");
+    assert_eq!(info.last_checked_at, DateTime::<Utc>::UNIX_EPOCH);
+    assert_eq!(
+        (
+            info.latest_version.as_str(),
+            info.dismissed_version.as_deref()
+        ),
+        ("999.0.0", Some("999.0.0"))
+    );
+}