Persist response token usage in rollout history (#41912)

## Why

Resumed threads need to continue per-turn and per-thread token totals without scanning arbitrarily far beyond the latest compaction checkpoint.

## What changed

- Add durable `TokenUsageRecord` rollout items with response, turn, thread, session, and root-turn attribution.
- Restore accumulated usage on resume and snapshot the latest record plus the compaction response ID in compaction checkpoints.
- Preserve root-turn lineage in persisted turn context, while ensuring forked child threads start with their own usage totals.

## Testing

- Cover usage accumulation across multiple responses and resumed turns.
- Cover local and remote compaction checkpoints, invalid remote compaction output, rollout reconstruction, and fork isolation.

GitOrigin-RevId: ef9e0c4a9102a08a2c382be4cdac68c84353c90a

codex-rs/external-agent-migration/src/sessions/append.rs

@@ -219,6 +219,7 @@ fn source_model_items(items: &[RolloutItem]) -> Option<Vec<SourceModelItem<'_>>>
         match item {
             RolloutItem::SessionMeta(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
+            | RolloutItem::TokenUsageRecord(_)
             | RolloutItem::RealtimeItem(_) => {}
             RolloutItem::ResponseItem(response_item) => {
                 model_items.push(SourceModelItem {
@@ -257,6 +258,7 @@ fn history_model_items(items: &[RolloutItem]) -> Option<Vec<&ResponseItem>> {
         match item {
             RolloutItem::SessionMeta(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
+            | RolloutItem::TokenUsageRecord(_)
             | RolloutItem::RealtimeItem(_)
             | RolloutItem::SecurityRiskScore(_) => {}
             RolloutItem::ResponseItem(response_item) => model_items.push(&response_item.item),