Add turn-scoped context contributions (#28911)

## Summary
- keep context injection on a single ContextContributor trait
- split context injection into thread-scoped and turn-scoped
contribution methods
- wire turn-scoped fragments into initial context assembly so extensions
can contribute context from turn-local state

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

@@ -12,12 +12,14 @@ use codex_extension_api::ExtensionEventSink;
 use codex_extension_api::ExtensionFuture;
 use codex_extension_api::ExtensionRegistryBuilder;
 use codex_extension_api::PromptFragment;
+use codex_extension_api::PromptSlot;
 use codex_extension_api::ThreadLifecycleContributor;
 use codex_extension_api::TokenUsageContributor;
 use codex_extension_api::ToolCall;
 use codex_extension_api::ToolContributor;
 use codex_extension_api::ToolExecutor;
 use codex_extension_api::ToolLifecycleContributor;
+use codex_extension_api::TurnContextContributionInput;
 use codex_extension_api::TurnInputContext;
 use codex_extension_api::TurnInputContributor;
 use codex_extension_api::TurnItemContributor;
@@ -34,7 +36,7 @@ use pretty_assertions::assert_eq;
 struct AllContributors;
 
 impl ContextContributor for AllContributors {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         _session_store: &'a ExtensionData,
         _thread_store: &'a ExtensionData,
@@ -147,7 +149,7 @@ async fn build_round_trips_every_contributor_category() {
 struct NamedContextContributor(&'static str);
 
 impl ContextContributor for NamedContextContributor {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         _session_store: &'a ExtensionData,
         _thread_store: &'a ExtensionData,
@@ -158,6 +160,20 @@ impl ContextContributor for NamedContextContributor {
     }
 }
 
+struct NamedTurnContextContributor(&'static str);
+
+impl ContextContributor for NamedTurnContextContributor {
+    fn contribute_turn_context<'a>(
+        &'a self,
+        _input: TurnContextContributionInput<'a>,
+    ) -> ExtensionFuture<'a, Vec<PromptFragment>> {
+        Box::pin(std::future::ready(vec![PromptFragment::new(
+            PromptSlot::ContextualUser,
+            self.0,
+        )]))
+    }
+}
+
 struct RecordingTurnItemContributor {
     name: &'static str,
     calls: Arc<Mutex<Vec<&'static str>>>,
@@ -186,6 +202,8 @@ async fn contributors_preserve_registration_order() {
     let mut builder = ExtensionRegistryBuilder::<()>::new();
     builder.prompt_contributor(Arc::new(NamedContextContributor("first")));
     builder.prompt_contributor(Arc::new(NamedContextContributor("second")));
+    builder.prompt_contributor(Arc::new(NamedTurnContextContributor("turn-first")));
+    builder.prompt_contributor(Arc::new(NamedTurnContextContributor("turn-second")));
     for name in ["first", "second"] {
         builder.turn_item_contributor(Arc::new(RecordingTurnItemContributor {
             name,
@@ -199,7 +217,25 @@ async fn contributors_preserve_registration_order() {
 
     let mut fragments = Vec::new();
     for contributor in registry.context_contributors() {
-        fragments.extend(contributor.contribute(&session_store, &thread_store).await);
+        fragments.extend(
+            contributor
+                .contribute_thread_context(&session_store, &thread_store)
+                .await,
+        );
+    }
+    for contributor in registry.context_contributors() {
+        fragments.extend(
+            contributor
+                .contribute_turn_context(TurnContextContributionInput {
+                    thread_id: codex_protocol::ThreadId::default(),
+                    turn_id: turn_store.level_id(),
+                    session_store: &session_store,
+                    thread_store: &thread_store,
+                    turn_store: &turn_store,
+                    model_context_window: Some(123),
+                })
+                .await,
+        );
     }
     let mut item = TurnItem::HookPrompt(HookPromptItem {
         id: "item".to_string(),
@@ -217,6 +253,8 @@ async fn contributors_preserve_registration_order() {
         vec![
             PromptFragment::developer_policy("first"),
             PromptFragment::developer_policy("second"),
+            PromptFragment::new(PromptSlot::ContextualUser, "turn-first"),
+            PromptFragment::new(PromptSlot::ContextualUser, "turn-second"),
         ]
     );
     assert_eq!(