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/app-server/src/bespoke_event_handling.rs

@@ -3331,6 +3331,7 @@ mod tests {
             Event {
                 id: "turn-1".to_string(),
                 msg: EventMsg::ItemCompleted(ItemCompletedEvent {
+                    content_token_count: None,
                     thread_id: conversation_id,
                     turn_id: "turn-1".to_string(),
                     item: CoreTurnItem::SubAgentActivity(SubAgentActivityItem {
@@ -3503,6 +3504,7 @@ mod tests {
             state.track_current_turn_event(
                 &event_turn_id,
                 &EventMsg::ItemCompleted(ItemCompletedEvent {
+                    content_token_count: None,
                     thread_id: conversation_id,
                     turn_id: event_turn_id.clone(),
                     item: CoreTurnItem::AgentMessage(CoreAgentMessageItem {
@@ -3527,6 +3529,7 @@ mod tests {
             state.track_current_turn_event(
                 &event_turn_id,
                 &EventMsg::ItemCompleted(ItemCompletedEvent {
+                    content_token_count: None,
                     thread_id: conversation_id,
                     turn_id: event_turn_id.clone(),
                     item: CoreTurnItem::AgentMessage(CoreAgentMessageItem {