Filter out reasoning items from previous turns (#5857)

Reduces request size and prevents 400 errors when switching between API
orgs.

Based on Responses API behavior described in
https://cookbook.openai.com/examples/responses_api/reasoning_items#caching

codex-rs/core/src/state/session.rs

@@ -34,10 +34,6 @@ impl SessionState {
         self.history.record_items(items)
     }
 
-    pub(crate) fn history_snapshot(&mut self) -> Vec<ResponseItem> {
-        self.history.get_history()
-    }
-
     pub(crate) fn clone_history(&self) -> ConversationHistory {
         self.history.clone()
     }