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/memories/src/extension.rs

@@ -48,7 +48,7 @@ impl MemoriesExtensionConfig {
 }
 
 impl ContextContributor for MemoriesExtension {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         _session_store: &'a ExtensionData,
         thread_store: &'a ExtensionData,

codex-rs/ext/memories/src/tests.rs

@@ -181,7 +181,7 @@ async fn prompt_contribution_uses_memory_summary_when_enabled() {
     });
 
     let fragments = extension
-        .contribute(&ExtensionData::new("session"), &thread_store)
+        .contribute_thread_context(&ExtensionData::new("session"), &thread_store)
         .await;
 
     assert_eq!(fragments.len(), 1);