Moving token_info to ConversationHistory (#5581)

I want to centralize input processing and management to
`ConversationHistory`. This would need `ConversationHistory` to have
access to `token_info` (i.e. preventing adding a big input to the
history). Besides, it makes more sense to have it on
`ConversationHistory` than `state`.

codex-rs/core/src/conversation_history.rs

@@ -1,5 +1,7 @@
 use codex_protocol::models::FunctionCallOutputPayload;
 use codex_protocol::models::ResponseItem;
+use codex_protocol::protocol::TokenUsage;
+use codex_protocol::protocol::TokenUsageInfo;
 use tracing::error;
 
 /// Transcript of conversation history
@@ -7,11 +9,28 @@ use tracing::error;
 pub(crate) struct ConversationHistory {
     /// The oldest items are at the beginning of the vector.
     items: Vec<ResponseItem>,
+    token_info: Option<TokenUsageInfo>,
 }
 
 impl ConversationHistory {
     pub(crate) fn new() -> Self {
-        Self { items: Vec::new() }
+        Self {
+            items: Vec::new(),
+            token_info: TokenUsageInfo::new_or_append(&None, &None, None),
+        }
+    }
+
+    pub(crate) fn token_info(&self) -> Option<TokenUsageInfo> {
+        self.token_info.clone()
+    }
+
+    pub(crate) fn set_token_usage_full(&mut self, context_window: i64) {
+        match &mut self.token_info {
+            Some(info) => info.fill_to_context_window(context_window),
+            None => {
+                self.token_info = Some(TokenUsageInfo::full_context_window(context_window));
+            }
+        }
     }
 
     /// `items` is ordered from oldest to newest.
@@ -301,6 +320,18 @@ impl ConversationHistory {
             self.items.remove(pos);
         }
     }
+
+    pub(crate) fn update_token_info(
+        &mut self,
+        usage: &TokenUsage,
+        model_context_window: Option<i64>,
+    ) {
+        self.token_info = TokenUsageInfo::new_or_append(
+            &self.token_info,
+            &Some(usage.clone()),
+            model_context_window,
+        );
+    }
 }
 
 #[inline]