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/src/lib.rs

@@ -54,6 +54,7 @@ pub use contributors::ToolLifecycleContributor;
 pub use contributors::ToolLifecycleFuture;
 pub use contributors::ToolStartInput;
 pub use contributors::TurnAbortInput;
+pub use contributors::TurnContextContributionInput;
 pub use contributors::TurnErrorInput;
 pub use contributors::TurnInputContext;
 pub use contributors::TurnInputContributor;