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

@@ -26,11 +26,11 @@ pub use turn_lifecycle::TurnStopInput;
 
 /// Extension contribution that adds prompt fragments during prompt assembly.
 pub trait ContextContributor: Send + Sync {
-    fn contribute(
-        &self,
-        session_store: &ExtensionData,
-        thread_store: &ExtensionData,
-    ) -> Vec<PromptFragment>;
+    fn contribute<'a>(
+        &'a self,
+        session_store: &'a ExtensionData,
+        thread_store: &'a ExtensionData,
+    ) -> std::pin::Pin<Box<dyn Future<Output = Vec<PromptFragment>> + Send + 'a>>;
 }
 
 /// Contributor for host-owned thread lifecycle gates.