Add ordinals to paginated rollout records (#32332)

## Why

Paginated thread history needs durable ordering so consumers can process a rollout suffix without rebuilding all earlier history.

## What changed

- Add optional, zero-based ordinals to `RolloutLine` records in paginated rollouts while leaving legacy rollout serialization unchanged.
- Continue ordinals from the last valid record when appending or resuming, including after gaps or an incomplete tail, and reject overflow without appending.
- Add a stateless `project_rollout_line` helper that maps canonical turn lifecycle and completed-item records into thread-history change sets.

## Testing

- Cover ordinal assignment, legacy compatibility, resume and tail recovery, overflow handling, and thread-history projection for completed, failed, and interrupted turns.

GitOrigin-RevId: 3a9bb6cd2a1f674a9f154342e96e9aa3d8330781

codex-rs/app-server/src/request_processors/thread_processor_tests.rs

@@ -1015,6 +1015,7 @@ mod thread_processor_behavior_tests {
 
         let line = RolloutLine {
             timestamp: timestamp.clone(),
+            ordinal: None,
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: session_meta.clone(),
                 git: None,
@@ -1082,6 +1083,7 @@ mod thread_processor_behavior_tests {
 
         let line = RolloutLine {
             timestamp,
+            ordinal: None,
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: session_meta,
                 git: None,
@@ -1124,6 +1126,7 @@ mod thread_processor_behavior_tests {
 
         let line = RolloutLine {
             timestamp,
+            ordinal: None,
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: session_meta,
                 git: None,