test: update conversation assembly coverage and docs

packages/client/test-runtime/src/fixtures.ts

@@ -2,6 +2,7 @@
 import type {
   ConversationSnapshot, ISession, SessionId, SessionSummary, WorkspaceListState,
 } from '@deepseek-ai/dsh-client-runtime/client'
+import { EMPTY_CHAT_SNAPSHOT } from '@deepseek-ai/dsh-client-runtime/client'
 
 /**
  * Fixture overrides for the session behavior face: any subset of the
@@ -45,6 +46,7 @@ export interface SessionFixture {
 export function conversationSnapshot(sessionId: SessionId): ConversationSnapshot {
   return {
     sessionId,
+    chat: EMPTY_CHAT_SNAPSHOT,
     nodes: [],
     turnTimings: new Map(),
     turnEnds: new Map(),