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/examples/enabled_extensions.rs

@@ -74,7 +74,11 @@ async fn contribute_prompt(
 ) -> Vec<codex_extension_api::PromptFragment> {
     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,
+        );
     }
     fragments
 }

codex-rs/ext/extension-api/examples/enabled_extensions/shared_state_extension.rs

@@ -17,7 +17,7 @@ pub fn install(registry: &mut ExtensionRegistryBuilder<()>) {
 struct StyleContributor;
 
 impl ContextContributor for StyleContributor {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         session_store: &'a ExtensionData,
         thread_store: &'a ExtensionData,
@@ -37,7 +37,7 @@ impl ContextContributor for StyleContributor {
 struct UsageContributor;
 
 impl ContextContributor for UsageContributor {
-    fn contribute<'a>(
+    fn contribute_thread_context<'a>(
         &'a self,
         session_store: &'a ExtensionData,
         thread_store: &'a ExtensionData,