Retain user instructions in guardian thread context (#42762)

## Why

Guardian review needs the original user instructions alongside verified answers, even after compaction, resume, or rollback. A summarized or legacy checkpoint cannot establish a complete instruction history.

## What changed

- Capture text user messages in retained context when `guardian_thread_context` is enabled, preserving their message identity through compaction and replay.
- Keep retained instructions and verified answers in arrival order, with independent size and completeness tracking for each family.
- Roll back retained evidence at the exact user-message boundary, including steers that share a turn ID, and apply the same semantics during rollout migration.
- Treat checkpoints without retained user messages as incomplete while preserving their verified answers.

## Testing

- Cover capture, compaction, resume, rollback, child forks, legacy checkpoints, storage bounds, and rollout migration.

GitOrigin-RevId: 72706731b54e5ac8645db61b446d2401afb42235

codex-rs/core/src/compact_tests.rs

@@ -66,6 +66,7 @@ fn user_message(text: &str) -> ResponseItem {
 
 fn compacted_user_message(text: &str) -> CompactedUserMessage {
     CompactedUserMessage {
+        id: None,
         message: text.to_string(),
         internal_chat_message_metadata_passthrough: None,
         harness_metadata: None,
@@ -129,7 +130,13 @@ fn collect_user_messages_extracts_user_text_only() {
 
     let collected = collect_user_messages(&items);
 
-    assert_eq!(vec![compacted_user_message("first")], collected);
+    assert_eq!(
+        vec![CompactedUserMessage {
+            id: Some(ResponseItemId::with_suffix("msg", "user")),
+            ..compacted_user_message("first")
+        }],
+        collected,
+    );
 }
 
 #[test]
@@ -142,10 +149,11 @@ fn collect_annotated_user_messages_extracts_user_text_only() {
         ResponseItemEnvelope::new(ResponseItem::Other),
     ];
 
-    let collected = collect_annotated_user_messages(&items);
+    let collected = collect_annotated_user_messages(&items, CompactedMessageIdentity::Preserve);
 
     assert_eq!(
         vec![CompactedUserMessage {
+            id: None,
             message: "first".to_string(),
             internal_chat_message_metadata_passthrough: None,
             harness_metadata: Some(CodexHarnessMetadata::default()),
@@ -223,6 +231,7 @@ fn build_token_limited_compacted_history_truncates_overlong_user_messages() {
     let max_tokens = 16;
     let big = "word ".repeat(200);
     let user_message = CompactedUserMessage {
+        id: Some(ResponseItemId::with_suffix("msg", "long-user")),
         message: big.clone(),
         internal_chat_message_metadata_passthrough: None,
         harness_metadata: Some(CodexHarnessMetadata::default()),
@@ -261,6 +270,7 @@ fn build_token_limited_compacted_history_truncates_overlong_user_messages() {
         other => panic!("unexpected item in history: {other:?}"),
     };
     assert_eq!(summary_text, "SUMMARY");
+    assert_eq!(history[0].id(), user_message.id.as_ref());
     assert_eq!(history[0].metadata, Some(CodexHarnessMetadata::default()));
     assert_eq!(history[1].metadata, None);
 }
@@ -292,6 +302,7 @@ fn build_compacted_history_preserves_user_message_passthrough_metadata() {
     let history = build_compacted_history(
         Vec::new(),
         &[CompactedUserMessage {
+            id: Some(ResponseItemId::with_suffix("msg", "user")),
             message: "first user message".to_string(),
             internal_chat_message_metadata_passthrough: Some(
                 InternalChatMessageMetadataPassthrough {
@@ -314,7 +325,7 @@ fn build_compacted_history_preserves_user_message_passthrough_metadata() {
         vec![
             ResponseItemEnvelope {
                 item: ResponseItem::Message {
-                    id: None,
+                    id: Some(ResponseItemId::with_suffix("msg", "user")),
                     role: "user".to_string(),
                     content: vec![ContentItem::InputText {
                         text: "first user message".to_string(),