feat: add token usage contributor hook (#22485)

## Why

Extensions need a stable place to observe token accounting after Codex
folds model-provider usage into the session's cached `TokenUsageInfo`.
Without a contributor hook, extension-owned features that need last-turn
or cumulative token usage have to duplicate session plumbing or infer
state from client-facing `TokenCount` notifications.

## What changed

- Added `TokenUsageContributor` to `codex-extension-api`, passing
session/thread `ExtensionData`, `ThreadId`, turn id, and the current
`TokenUsageInfo`.
- Added registry builder/storage support for token-usage contributors.
- Invoked registered contributors from
`Session::record_token_usage_info` after the session token cache is
updated and before the client `TokenCount` notification is emitted.

## Testing

- Added `record_token_usage_info_notifies_extension_contributors`,
covering cumulative token usage updates and access to both extension
stores.

codex-rs/ext/extension-api/src/contributors.rs

@@ -1,8 +1,10 @@
 use std::future::Future;
 use std::sync::Arc;
 
+use codex_protocol::ThreadId;
 use codex_protocol::items::TurnItem;
 use codex_protocol::protocol::ReviewDecision;
+use codex_protocol::protocol::TokenUsageInfo;
 
 use crate::ExtensionData;
 
@@ -66,6 +68,24 @@ pub trait TurnLifecycleContributor: Send + Sync {
     fn on_turn_abort(&self, _input: TurnAbortInput<'_>) {}
 }
 
+/// Contributor for token usage checkpoints reported by the model provider.
+///
+/// Implementations should keep this callback cheap. The host calls it after
+/// updating cached token usage and before emitting the corresponding client
+/// token-count notification.
+pub trait TokenUsageContributor: Send + Sync {
+    /// Called each time the host records token usage from a model response.
+    fn on_token_usage(
+        &self,
+        _session_store: &ExtensionData,
+        _thread_store: &ExtensionData,
+        _thread_id: ThreadId,
+        _turn_id: &str,
+        _token_usage: &TokenUsageInfo,
+    ) {
+    }
+}
+
 /// Extension contribution that exposes native tools owned by a feature.
 pub trait ToolContributor: Send + Sync {
     /// Returns the native tools visible for the supplied extension stores.

codex-rs/ext/extension-api/src/lib.rs

@@ -25,6 +25,7 @@ pub use contributors::ThreadLifecycleContributor;
 pub use contributors::ThreadResumeInput;
 pub use contributors::ThreadStartInput;
 pub use contributors::ThreadStopInput;
+pub use contributors::TokenUsageContributor;
 pub use contributors::ToolContributor;
 pub use contributors::TurnAbortInput;
 pub use contributors::TurnItemContributionFuture;

codex-rs/ext/extension-api/src/registry.rs

@@ -5,6 +5,7 @@ use crate::ApprovalReviewFuture;
 use crate::ContextContributor;
 use crate::ExtensionData;
 use crate::ThreadLifecycleContributor;
+use crate::TokenUsageContributor;
 use crate::ToolContributor;
 use crate::TurnItemContributor;
 use crate::TurnLifecycleContributor;
@@ -13,6 +14,7 @@ use crate::TurnLifecycleContributor;
 pub struct ExtensionRegistryBuilder<C> {
     thread_lifecycle_contributors: Vec<Arc<dyn ThreadLifecycleContributor<C>>>,
     turn_lifecycle_contributors: Vec<Arc<dyn TurnLifecycleContributor>>,
+    token_usage_contributors: Vec<Arc<dyn TokenUsageContributor>>,
     context_contributors: Vec<Arc<dyn ContextContributor>>,
     tool_contributors: Vec<Arc<dyn ToolContributor>>,
     turn_item_contributors: Vec<Arc<dyn TurnItemContributor>>,
@@ -24,6 +26,7 @@ impl<C> Default for ExtensionRegistryBuilder<C> {
         Self {
             thread_lifecycle_contributors: Vec::new(),
             turn_lifecycle_contributors: Vec::new(),
+            token_usage_contributors: Vec::new(),
             approval_review_contributors: Vec::new(),
             context_contributors: Vec::new(),
             tool_contributors: Vec::new(),
@@ -56,6 +59,11 @@ impl<C> ExtensionRegistryBuilder<C> {
         self.turn_lifecycle_contributors.push(contributor);
     }
 
+    /// Registers one token-usage contributor.
+    pub fn token_usage_contributor(&mut self, contributor: Arc<dyn TokenUsageContributor>) {
+        self.token_usage_contributors.push(contributor);
+    }
+
     /// Registers one prompt contributor.
     pub fn prompt_contributor(&mut self, contributor: Arc<dyn ContextContributor>) {
         self.context_contributors.push(contributor);
@@ -76,6 +84,7 @@ impl<C> ExtensionRegistryBuilder<C> {
         ExtensionRegistry {
             thread_lifecycle_contributors: self.thread_lifecycle_contributors,
             turn_lifecycle_contributors: self.turn_lifecycle_contributors,
+            token_usage_contributors: self.token_usage_contributors,
             approval_review_contributors: self.approval_review_contributors,
             context_contributors: self.context_contributors,
             tool_contributors: self.tool_contributors,
@@ -88,6 +97,7 @@ impl<C> ExtensionRegistryBuilder<C> {
 pub struct ExtensionRegistry<C> {
     thread_lifecycle_contributors: Vec<Arc<dyn ThreadLifecycleContributor<C>>>,
     turn_lifecycle_contributors: Vec<Arc<dyn TurnLifecycleContributor>>,
+    token_usage_contributors: Vec<Arc<dyn TokenUsageContributor>>,
     context_contributors: Vec<Arc<dyn ContextContributor>>,
     tool_contributors: Vec<Arc<dyn ToolContributor>>,
     turn_item_contributors: Vec<Arc<dyn TurnItemContributor>>,
@@ -105,6 +115,11 @@ impl<C> ExtensionRegistry<C> {
         &self.turn_lifecycle_contributors
     }
 
+    /// Returns the registered token-usage contributors.
+    pub fn token_usage_contributors(&self) -> &[Arc<dyn TokenUsageContributor>] {
+        &self.token_usage_contributors
+    }
+
     /// Claims the first rendered approval-review prompt accepted by an
     /// installed contributor.
     pub fn approval_review<'a>(