[codex-analytics] feature plumbing and emittance (#16640)

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[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16640).
* #16870
* #16706
* #16641
* __->__ #16640

codex-rs/core/src/thread_rollout_truncation.rs

@@ -8,9 +8,21 @@ use crate::event_mapping;
 use codex_protocol::items::TurnItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::InterAgentCommunication;
 use codex_protocol::protocol::RolloutItem;
 
+pub(crate) fn initial_history_has_prior_user_turns(conversation_history: &InitialHistory) -> bool {
+    conversation_history.scan_rollout_items(rollout_item_is_user_turn_boundary)
+}
+
+fn rollout_item_is_user_turn_boundary(item: &RolloutItem) -> bool {
+    match item {
+        RolloutItem::ResponseItem(item) => is_user_turn_boundary(item),
+        _ => false,
+    }
+}
+
 /// Return the indices of user message boundaries in a rollout.
 ///
 /// A user message boundary is a `RolloutItem::ResponseItem(ResponseItem::Message { .. })`