Preserve Guardian history across thread reconstruction (#42065)

## Why

Guardian review evidence must remain available after a compacted thread is resumed or forked, while rolled-back or parent-local evidence must not grant authorization in a different context.

## What changed

- Store the bounded, model-invisible Guardian transcript alongside compacted rollout history and restore it from the newest surviving checkpoint during replay.
- Trim Guardian history at rollback boundaries, clearing it when the boundary has already been evicted.
- Preserve the checkpoint for user-initiated forks, but remove it when spawning a subagent so parent review evidence is not inherited.
- Keep the new rollout field optional for compatibility with existing compacted records and legacy readers.

## Testing

- Cover Guardian history across compaction, restart, paginated and pathless stores, user forks, rollback, bounded replay, serialization, and subagent forks.

GitOrigin-RevId: 6ab076d5e0c3aac4d6a41ebd50ba2b200c90213e

codex-rs/thread-store/src/local/model_context_tests.rs

@@ -759,6 +759,7 @@ fn compacted(message: &str, replacement_history: Option<Vec<ResponseItem>>) -> R
         message: message.to_string(),
         replacement_history: replacement_history
             .map(|items| items.into_iter().map(Into::into).collect()),
+        guardian_history: None,
         mcp_resource_origins: None,
         window_number: Some(1),
         first_window_id: None,

codex-rs/thread-store/src/local/rollout_migration_tests.rs

@@ -230,6 +230,7 @@ fn compacted(replacement_history: Vec<ResponseItem>) -> RolloutItem {
     RolloutItem::Compacted(CompactedItem {
         message: "checkpoint".to_string(),
         replacement_history: Some(replacement_history.into_iter().map(Into::into).collect()),
+        guardian_history: None,
         mcp_resource_origins: None,
         window_number: Some(1),
         first_window_id: None,
@@ -1408,6 +1409,7 @@ async fn migration_compacts_subagent_prefix_and_does_not_project_it() {
             RolloutItem::Compacted(CompactedItem {
                 message: "superseded checkpoint".repeat(1024),
                 replacement_history: Some(Vec::new()),
+                guardian_history: None,
                 mcp_resource_origins: None,
                 window_number: Some(1),
                 first_window_id: None,
@@ -1430,6 +1432,7 @@ async fn migration_compacts_subagent_prefix_and_does_not_project_it() {
                     }
                     .into(),
                 ]),
+                guardian_history: None,
                 mcp_resource_origins: None,
                 window_number: Some(2),
                 first_window_id: None,