Make context contributors async (#22491)

## Summary
- make ContextContributor return a boxed Send future
- await context contributors during initial context assembly
- update existing contributors and extension-api examples for the async
contract

## Testing
- cargo test -p codex-extension-api --examples
- cargo test -p codex-git-attribution
- cargo test -p codex-core
build_initial_context_includes_git_attribution_from_extensions --
--nocapture
- cargo test -p codex-core
build_initial_context_omits_git_attribution_when_feature_is_disabled --
--nocapture
- cargo test -p codex-core (fails in unrelated
agent::control::tests::spawn_agent_fork_last_n_turns_keeps_only_recent_turns
stack overflow)
- just fix -p codex-extension-api
- just fix -p codex-git-attribution
- just fix -p codex-core
- cargo clippy -p codex-extension-api --examples

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

@@ -2694,11 +2694,15 @@ impl Session {
         {
             developer_sections.push(plugin_instructions.render());
         }
-        for contributor in self.services.extensions.context_contributors() {
-            for fragment in contributor.contribute(
-                &self.services.session_extension_data,
-                &self.services.thread_extension_data,
-            ) {
+        let context_contributors = self.services.extensions.context_contributors().to_vec();
+        for contributor in context_contributors {
+            for fragment in contributor
+                .contribute(
+                    &self.services.session_extension_data,
+                    &self.services.thread_extension_data,
+                )
+                .await
+            {
                 match fragment.slot() {
                     PromptSlot::DeveloperPolicy | PromptSlot::DeveloperCapabilities => {
                         developer_sections.push(fragment.text().to_string());

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

@@ -6352,21 +6352,25 @@ struct GitAttributionTestContributor;
 struct GitAttributionTestState;
 
 impl codex_extension_api::ContextContributor for GitAttributionTestContributor {
-    fn contribute(
-        &self,
-        _session_store: &codex_extension_api::ExtensionData,
-        thread_store: &codex_extension_api::ExtensionData,
-    ) -> Vec<codex_extension_api::PromptFragment> {
-        thread_store
-            .get::<GitAttributionTestState>()
-            .is_some()
-            .then(|| {
-                codex_extension_api::PromptFragment::developer_policy(
-                    "git attribution extension enabled",
-                )
-            })
-            .into_iter()
-            .collect()
+    fn contribute<'a>(
+        &'a self,
+        _session_store: &'a codex_extension_api::ExtensionData,
+        thread_store: &'a codex_extension_api::ExtensionData,
+    ) -> std::pin::Pin<
+        Box<dyn std::future::Future<Output = Vec<codex_extension_api::PromptFragment>> + Send + 'a>,
+    > {
+        Box::pin(async move {
+            thread_store
+                .get::<GitAttributionTestState>()
+                .is_some()
+                .then(|| {
+                    codex_extension_api::PromptFragment::developer_policy(
+                        "git attribution extension enabled",
+                    )
+                })
+                .into_iter()
+                .collect()
+        })
     }
 }