Include terminal errors in turn completion events (#32280)

## What changed

- Add an optional `error` payload to `TurnCompleteEvent` and omit it when a turn completes without an error.
- Preserve the full terminal `ErrorEvent` through the turn lifecycle so the completion event includes the same error details emitted separately by `EventMsg::Error`.

## Testing

- Extend the stream-error integration test to verify that `TurnCompleteEvent` contains the emitted error and that the next turn can still proceed.

GitOrigin-RevId: 8d32942a44132763076130102d8b89ab356d59d0

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

@@ -9,6 +9,7 @@ use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::protocol::ErrorEvent;
 use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::ThreadHistoryMode;
 use codex_protocol::protocol::TurnEnvironmentSelection;
@@ -140,7 +141,7 @@ pub struct TurnContext {
     pub(crate) extension_data: Arc<codex_extension_api::ExtensionData>,
     pub(crate) turn_skills: TurnSkillsContext,
     pub(crate) turn_timing_state: Arc<TurnTimingState>,
-    pub(crate) terminal_error: Arc<Mutex<Option<String>>>,
+    pub(crate) terminal_error: Arc<Mutex<Option<ErrorEvent>>>,
     pub(crate) server_model_warning_emitted: AtomicBool,
     pub(crate) model_verification_emitted: AtomicBool,
 }