fix: stabilize streamed message rendering

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/sidecar.ts

@@ -133,6 +133,7 @@ export interface TurnDeltaEvent {
   messageId: string;
   authorName: string;
   contentDelta: string;
+  content?: string;
 }
 
 export interface TurnCompleteEvent {

src/shared/domain/event.ts

@@ -15,6 +15,7 @@ export interface SessionEventRecord {
   messageId?: string;
   authorName?: string;
   contentDelta?: string;
+  content?: string;
   activityType?: SessionActivityType;
   agentId?: string;
   agentName?: string;