Expose assistant content token counts in completion notifications (#59041)

## What changed

Add optional `contentTokenCount` metadata to `item/completed` notifications, carrying the producer-supplied token count for each completed assistant message in default and plan modes. Counts include any sampled footer and are `null` when unavailable.

Keep this telemetry separate from stored thread items and omit it from subsequent model requests. Update the protocol schemas and generated TypeScript and Python types.

## Testing

Add coverage for per-message counts, including zero, in default and plan modes; missing and malformed metadata; backward-compatible notification decoding; and exclusion from subsequent model requests.

GitOrigin-RevId: 35973037a42650fa2d7569da8e88cb81215cf971

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

@@ -2329,6 +2329,7 @@ mod tests {
             .await
             .expect("create paginated thread");
         let paginated_item = RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+            content_token_count: None,
             thread_id,
             turn_id: "turn-1".to_string(),
             item: TurnItem::UserMessage(UserMessageItem {

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

@@ -720,6 +720,7 @@ fn contextual_user_message() -> RolloutItem {
 
 fn completed_user_message(turn_id: &str, message: &str) -> RolloutItem {
     RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+        content_token_count: None,
         thread_id: codex_protocol::ThreadId::from_string("00000000-0000-0000-0000-000000000000")
             .expect("fixture thread id"),
         turn_id: turn_id.to_string(),

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

@@ -936,6 +936,7 @@ mod tests {
         codex_rollout::append_rollout_item_to_path(
             &rollout_path,
             &RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+                content_token_count: None,
                 thread_id,
                 turn_id: "turn-1".to_string(),
                 item: TurnItem::UserMessage(UserMessageItem::new(&[UserInput::Text {

codex-rs/thread-store/src/local/rollout_migration/canonicalizer.rs

@@ -422,6 +422,7 @@ impl LegacyRolloutCanonicalizer {
             writer,
             timestamp,
             RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+                content_token_count: None,
                 thread_id: self.thread_id,
                 turn_id,
                 item,

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

@@ -222,6 +222,7 @@ fn exec_completion(turn_id: &str, call_id: &str, output: &str) -> RolloutItem {
 
 fn item_completed(turn_id: &str, item_id: &str) -> RolloutItem {
     RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+        content_token_count: None,
         thread_id: ThreadId::new(),
         turn_id: turn_id.to_string(),
         item: TurnItem::Reasoning(ReasoningItem {

codex-rs/thread-store/src/local/tests/delegated_preview_tests.rs

@@ -29,6 +29,7 @@ async fn delegated_thread_is_listed_before_user_followup() {
         live_thread
             .append_items(&[RolloutItem::EventMsg(EventMsg::ItemCompleted(
                 ItemCompletedEvent {
+                    content_token_count: None,
                     thread_id,
                     turn_id: "delegated-turn".to_string(),
                     item: TurnItem::FunctionCallOutput(FunctionCallOutputItem {

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

@@ -1573,6 +1573,7 @@ async fn projection_preserves_exact_lifecycle_timestamps() {
     for (index, (started_at_ms, completed_at_ms)) in timestamps.iter().enumerate() {
         items.push(RolloutItem::EventMsg(EventMsg::ItemCompleted(
             ItemCompletedEvent {
+                content_token_count: None,
                 thread_id,
                 turn_id: "turn-1".to_string(),
                 item: TurnItem::UserMessage(UserMessageItem {
@@ -3171,6 +3172,7 @@ fn contains_user_message(items: &[RolloutItem], expected: &str) -> bool {
 
 fn completed_item(thread_id: ThreadId, turn_id: &str, item: TurnItem) -> RolloutItem {
     RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+        content_token_count: None,
         thread_id,
         turn_id: turn_id.to_string(),
         item,

codex-rs/thread-store/src/thread_metadata_sync.rs

@@ -538,6 +538,7 @@ mod tests {
                 thread_id,
                 vec![RolloutItem::EventMsg(EventMsg::ItemCompleted(
                     ItemCompletedEvent {
+                        content_token_count: None,
                         thread_id,
                         turn_id: "turn-1".to_string(),
                         item: TurnItem::UserMessage(UserMessageItem::new(&[UserInput::Text {
@@ -650,6 +651,7 @@ mod tests {
         let update = sync
             .observe_appended_items(&[RolloutItem::EventMsg(EventMsg::ItemCompleted(
                 ItemCompletedEvent {
+                    content_token_count: None,
                     thread_id,
                     turn_id: "turn-1".to_string(),
                     item: TurnItem::UserMessage(UserMessageItem::new(&[UserInput::Text {

codex-rs/thread-store/src/thread_metadata_sync_preview_tests.rs

@@ -27,6 +27,7 @@ fn delegated_output_emits_only_first_preview_in_live_patch() {
     );
     for (index, text) in ["first task", "later task"].into_iter().enumerate() {
         let item = RolloutItem::EventMsg(EventMsg::ItemCompleted(ItemCompletedEvent {
+            content_token_count: None,
             thread_id,
             turn_id: "turn".to_string(),
             item: TurnItem::FunctionCallOutput(FunctionCallOutputItem {