fix: keep cancelled turns as interrupted instead of merging into the next request

- Build runtime history per turn: a cancelled or failed reply keeps its partial
  text plus an interruption marker (or a marker-only placeholder), so its user
  message is never left unanswered and merged into the next request.
- Use the same history for send, manual compaction and Main compact validation.
- OpenCode: keep a cancelled prompt in the reused native session and prefix the
  next turn with a one-time synthetic interruption notice; failed prompts are
  still removed.
- Add an opt-in DeepSeek live test covering direct model and OpenCode.

src/shared/runtime-history.ts

@@ -0,0 +1,61 @@
+import type { ConversationMessage } from './assistant-contracts'
+
+export const cancelledResponseMarker =
+  '[This response was interrupted by the user before it finished.]'
+export const failedResponseMarker =
+  '[This response failed before it finished.]'
+
+type HistorySourceMessage = Pick<
+  ConversationMessage,
+  'id' | 'role' | 'content' | 'state' | 'terminalStatus' | 'status'
+>
+
+export type RuntimeHistoryMessage = {
+  id: string
+  role: ConversationMessage['role']
+  content: string
+}
+
+export function isCancelledMessage(
+  message: Pick<ConversationMessage, 'state' | 'terminalStatus' | 'status'>
+): boolean {
+  if (message.state !== 'error') return false
+  if (message.terminalStatus !== undefined) return message.terminalStatus === 'cancelled'
+  // Released snapshots lack terminalStatus. Only recognize exact old labels;
+  // every new terminal event supplies the structured value above.
+  return message.status === '请求已取消' || message.status === '已取消' ||
+    message.status === '远端 Runtime 请求已取消'
+}
+
+/**
+ * Builds the model-visible history from stored messages. A cancelled or failed
+ * reply stays in place with its partial text and an explicit marker, so its
+ * user message is never left unanswered and merged into the next turn.
+ */
+export function buildRuntimeHistory(
+  messages: readonly HistorySourceMessage[]
+): RuntimeHistoryMessage[] {
+  const history: RuntimeHistoryMessage[] = []
+  for (const message of messages) {
+    if (message.state === 'complete') {
+      if (message.content.trim()) {
+        history.push({ id: message.id, role: message.role, content: message.content })
+      }
+      continue
+    }
+    if (message.state !== 'error' || message.role !== 'assistant') continue
+    // Only a reply to a retained user turn needs a marker.
+    if (history.at(-1)?.role !== 'user') continue
+    const marker = isCancelledMessage(message)
+      ? cancelledResponseMarker
+      : failedResponseMarker
+    history.push({
+      id: message.id,
+      role: 'assistant',
+      content: message.content.trim()
+        ? `${message.content.trimEnd()}\n\n${marker}`
+        : marker
+    })
+  }
+  return history
+}