feat: add turn lifecycle contributors (#22480)

## Why

Extensions can already contribute prompt, tool, turn-item, and
thread-lifecycle behavior, but there was no explicit host-owned hook for
per-turn setup and cleanup. That makes extension-private turn state
awkward: an extension either has to stash it outside the turn lifecycle
or depend on core runtime objects.

This adds a small turn lifecycle boundary. Extensions receive stable
identifiers plus the existing session, thread, and turn `ExtensionData`
stores, while core keeps owning task scheduling, cancellation, and turn
teardown.

## What Changed

- Added `TurnLifecycleContributor` with `on_turn_start`, `on_turn_stop`,
and `on_turn_abort` callbacks in `codex-rs/ext/extension-api`.
- Added typed `TurnStartInput`, `TurnStopInput`, and `TurnAbortInput`
payloads that expose `thread_id`, `turn_id`, `session_store`,
`thread_store`, and `turn_store`.
- Registered and re-exported turn lifecycle contributors through
`ExtensionRegistry` and `ExtensionRegistryBuilder`.
- Wired `Session` to emit turn start, stop, and abort callbacks from the
existing turn/task lifecycle paths.
- Carried the turn-scoped `ExtensionData` through `RunningTask` and
`RemovedTask` so stop/abort callbacks receive the same turn store
created at turn start.

## Verification

- Not run locally.

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

@@ -9,6 +9,7 @@ use crate::ExtensionData;
 mod prompt;
 mod thread_lifecycle;
 mod tools;
+mod turn_lifecycle;
 
 pub use prompt::PromptFragment;
 pub use prompt::PromptSlot;
@@ -18,6 +19,9 @@ pub use thread_lifecycle::ThreadStopInput;
 pub use tools::ExtensionToolExecutor;
 pub use tools::ExtensionToolFuture;
 pub use tools::ExtensionToolOutput;
+pub use turn_lifecycle::TurnAbortInput;
+pub use turn_lifecycle::TurnStartInput;
+pub use turn_lifecycle::TurnStopInput;
 
 /// Extension contribution that adds prompt fragments during prompt assembly.
 pub trait ContextContributor: Send + Sync {
@@ -45,6 +49,23 @@ pub trait ThreadLifecycleContributor<C>: Send + Sync {
     fn on_thread_stop(&self, _input: ThreadStopInput<'_>) {}
 }
 
+/// Contributor for host-owned turn lifecycle gates.
+///
+/// Implementations should use these callbacks to seed, observe, or clear
+/// extension-private turn state. The host exposes stable identifiers and
+/// extension stores instead of core runtime objects.
+pub trait TurnLifecycleContributor: Send + Sync {
+    /// Called after turn-scoped extension stores are created, before the task
+    /// for the turn starts running.
+    fn on_turn_start(&self, _input: TurnStartInput<'_>) {}
+
+    /// Called before the host drops the completed turn runtime and turn store.
+    fn on_turn_stop(&self, _input: TurnStopInput<'_>) {}
+
+    /// Called after the host aborts a running turn.
+    fn on_turn_abort(&self, _input: TurnAbortInput<'_>) {}
+}
+
 /// 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/contributors/turn_lifecycle.rs

@@ -0,0 +1,48 @@
+use codex_protocol::ThreadId;
+use codex_protocol::protocol::TurnAbortReason;
+
+use crate::ExtensionData;
+
+/// Input supplied when the host starts a turn.
+pub struct TurnStartInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is starting.
+    pub turn_id: &'a str,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}
+
+/// Input supplied when the host completes a turn.
+pub struct TurnStopInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is stopping.
+    pub turn_id: &'a str,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}
+
+/// Input supplied when the host aborts a turn.
+pub struct TurnAbortInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is aborting.
+    pub turn_id: &'a str,
+    /// Reason the host aborted the turn.
+    pub reason: TurnAbortReason,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}

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

@@ -26,8 +26,12 @@ pub use contributors::ThreadResumeInput;
 pub use contributors::ThreadStartInput;
 pub use contributors::ThreadStopInput;
 pub use contributors::ToolContributor;
+pub use contributors::TurnAbortInput;
 pub use contributors::TurnItemContributionFuture;
 pub use contributors::TurnItemContributor;
+pub use contributors::TurnLifecycleContributor;
+pub use contributors::TurnStartInput;
+pub use contributors::TurnStopInput;
 pub use registry::ExtensionRegistry;
 pub use registry::ExtensionRegistryBuilder;
 pub use registry::empty_extension_registry;

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

@@ -7,10 +7,12 @@ use crate::ExtensionData;
 use crate::ThreadLifecycleContributor;
 use crate::ToolContributor;
 use crate::TurnItemContributor;
+use crate::TurnLifecycleContributor;
 
 /// Mutable registry used while hosts register typed runtime contributions.
 pub struct ExtensionRegistryBuilder<C> {
     thread_lifecycle_contributors: Vec<Arc<dyn ThreadLifecycleContributor<C>>>,
+    turn_lifecycle_contributors: Vec<Arc<dyn TurnLifecycleContributor>>,
     context_contributors: Vec<Arc<dyn ContextContributor>>,
     tool_contributors: Vec<Arc<dyn ToolContributor>>,
     turn_item_contributors: Vec<Arc<dyn TurnItemContributor>>,
@@ -21,6 +23,7 @@ impl<C> Default for ExtensionRegistryBuilder<C> {
     fn default() -> Self {
         Self {
             thread_lifecycle_contributors: Vec::new(),
+            turn_lifecycle_contributors: Vec::new(),
             approval_review_contributors: Vec::new(),
             context_contributors: Vec::new(),
             tool_contributors: Vec::new(),
@@ -48,6 +51,11 @@ impl<C> ExtensionRegistryBuilder<C> {
         self.thread_lifecycle_contributors.push(contributor);
     }
 
+    /// Registers one turn-lifecycle contributor.
+    pub fn turn_lifecycle_contributor(&mut self, contributor: Arc<dyn TurnLifecycleContributor>) {
+        self.turn_lifecycle_contributors.push(contributor);
+    }
+
     /// Registers one prompt contributor.
     pub fn prompt_contributor(&mut self, contributor: Arc<dyn ContextContributor>) {
         self.context_contributors.push(contributor);
@@ -67,6 +75,7 @@ impl<C> ExtensionRegistryBuilder<C> {
     pub fn build(self) -> ExtensionRegistry<C> {
         ExtensionRegistry {
             thread_lifecycle_contributors: self.thread_lifecycle_contributors,
+            turn_lifecycle_contributors: self.turn_lifecycle_contributors,
             approval_review_contributors: self.approval_review_contributors,
             context_contributors: self.context_contributors,
             tool_contributors: self.tool_contributors,
@@ -78,6 +87,7 @@ impl<C> ExtensionRegistryBuilder<C> {
 /// Immutable typed registry produced after extensions are installed.
 pub struct ExtensionRegistry<C> {
     thread_lifecycle_contributors: Vec<Arc<dyn ThreadLifecycleContributor<C>>>,
+    turn_lifecycle_contributors: Vec<Arc<dyn TurnLifecycleContributor>>,
     context_contributors: Vec<Arc<dyn ContextContributor>>,
     tool_contributors: Vec<Arc<dyn ToolContributor>>,
     turn_item_contributors: Vec<Arc<dyn TurnItemContributor>>,
@@ -90,6 +100,11 @@ impl<C> ExtensionRegistry<C> {
         &self.thread_lifecycle_contributors
     }
 
+    /// Returns the registered turn-lifecycle contributors.
+    pub fn turn_lifecycle_contributors(&self) -> &[Arc<dyn TurnLifecycleContributor>] {
+        &self.turn_lifecycle_contributors
+    }
+
     /// Claims the first rendered approval-review prompt accepted by an
     /// installed contributor.
     pub fn approval_review<'a>(