fix: keep consecutive requests running and clear finished stop state

Wait for OpenCode session cancellation before reusing a conversation, and replace the session after unconfirmed cancellation. Preserve terminal message state when stale streaming snapshots arrive so finished conversations return to Send.

Validated with full production Electron UI and the configured real text model: queued requests complete, cancellation followed by immediate resend completes, and Stop clears. Focused tests: 362 passed; typecheck and targeted lint passed. Live dispatch counting limitations are recorded in the feature evidence.

Release note: Consecutive OpenCode requests no longer inherit a previous cancellation, and finished conversations no longer retain the Stop button after stale updates.

docs/features/assistant-workbar/runtime-interactions.md

@@ -92,6 +92,8 @@ DeepSeek Harness 按调用 ID 保留名称、输入和摘要，结束事件省
   正文、推理或工具进展会清除旧状态；子任务更新不覆盖父请求的重试提示。
 - 完成、失败、取消后保留真实终态，迟到的状态事件不能把请求恢复成处理中。底部状态点
   为静态，不凭请求尚未结束暗示仍有进展；会话列表闪点和发送、停止按钮保持原有行为。
+  持久化快照中的同一条消息仍为生成中时，也不能覆盖本地已收到的终态，即使快照的
+  会话更新时间更晚；否则连续发送或刷新会让已结束的请求重新显示停止按钮。
 - Continue、DeepSeek Harness 和远端 ACP 保留其已接入的处理与工具事件，不伪造未接入
   的原生重试状态。直连模型的重试等待与请求开始文案见
   [直连模型 UI 设计](../direct-model-agent/ui-design.md#6-错误与通知)；本次不改变任何