Move changing turn input functionalities to ConversationHistory (#5473)

We are doing some ad-hoc logic while dealing with conversation history.
Ideally, we shouldn't mutate `vec[responseitem]` manually at all and
should depend on `ConversationHistory` for those changes.

Those changes are:
- Adding input to the history
- Removing items from the history
- Correcting history

I am also adding some `error` logs for cases we shouldn't ideally face.
For example, we shouldn't be missing `toolcalls` or `outputs`. We
shouldn't hit `ContextWindowExceeded` while performing `compact`

This refactor will give us granular control over our context management.

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

@@ -36,8 +36,12 @@ impl SessionState {
         self.history.record_items(items)
     }
 
-    pub(crate) fn history_snapshot(&self) -> Vec<ResponseItem> {
-        self.history.contents()
+    pub(crate) fn history_snapshot(&mut self) -> Vec<ResponseItem> {
+        self.history.get_history()
+    }
+
+    pub(crate) fn clone_history(&self) -> ConversationHistory {
+        self.history.clone()
     }
 
     pub(crate) fn replace_history(&mut self, items: Vec<ResponseItem>) {