Support archiving and restoring cloud threads (#60635)

## What changed

- Add cloud archive and unarchive requests and support listing archived threads.
- Detach successfully archived threads and emit archive notifications. Report loaded cloud threads from live attachments rather than cached resume metadata, including after archive failures.
- Scope resume picker archive state and transcript previews by backend and thread so archiving a cloud session preserves a local session with the same ID.
- Limit cloud task archive cleanup to the selected thread, preserving child agents and their active session state, and update the confirmation text accordingly.

## Testing

Add regression coverage for archive acknowledgments, archived-thread discovery and restoration, attachment state after archive failures, same-ID sessions across backends, and preservation of attached child agents.

GitOrigin-RevId: bd155be85ee911375d4d326b18f9b5abe8e82b3d

codex-rs/cloud-client/src/client_tests.rs

@@ -109,6 +109,29 @@ async fn activation_sends_only_identity_and_excludes_turns() {
     assert_request(&mut fixture, "UpdateConfig", &[56, 1, 0x92, 1, 1, b't']).await;
 }
 
+#[tokio::test]
+async fn archive_addresses_the_requested_thread_and_requires_acknowledgment() {
+    let mut fixture = fixture_responses(vec![
+        response(vec![frame(&[])], tonic::Status::ok("")),
+        response(
+            vec![frame(&[])],
+            tonic::Status::permission_denied("synthetic denial"),
+        ),
+    ])
+    .await;
+
+    fixture.client.archive_thread("t").await.unwrap();
+    assert_request(&mut fixture, "ArchiveThread", b"\x0a\x01t").await;
+
+    let error = fixture.client.archive_thread("u").await.unwrap_err();
+    let MutationError::OutcomeUnknown(Error::Rpc(status)) = error else {
+        panic!("expected a remote archive failure");
+    };
+    assert_eq!(status.code, tonic::Code::PermissionDenied as i32);
+    assert_request(&mut fixture, "ArchiveThread", b"\x0a\x01u").await;
+    assert!(fixture.requests.try_recv().is_err());
+}
+
 #[tokio::test]
 async fn activation_preserves_native_settings_and_thread_metadata() {
     // Native Thread fields encoded independently of the client's wire types.