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/app-server/tests/suite/v2/thread_resume.rs

@@ -196,6 +196,7 @@ async fn thread_resume_paginated_model_context_preserves_original_metadata() ->
         &RolloutItem::Compacted(CompactedItem {
             message: "compacted history".to_string(),
             replacement_history: Some(Vec::new()),
+            guardian_history: None,
             mcp_resource_origins: None,
             window_number: Some(1),
             first_window_id: None,