fix(webui): 断连时流只收尾一次

上一条提交说「没有东西可修」是错的。CI 证伪了它。

## 双重收尾是真的

macOS CI 上 `finalises the stream exactly once` 报 `expected 2 to be 1`。

之前本地测出 1 是因为那个测试用 `setTimeout(250)` 等 `finally`。本地机器
快,250ms 内 `finally` 还没跑到,所以第二次 `return()` 没发生,断言看着是绿的。
CI 负载低,链路跑完了,它就露出来。

**我把这个当成了「没复现 = 不存在」**,而实际是「没复现 = 我等的那个时刻
还没到」。同一天里第三次栽在同一个地方:证据不足以支撑结论时下了结论。

## 修法:一个幂等门,两条路

`abort` listener 必须在:`next()` 永远不 settle 时,只有它能收尾那个迭代器。
`finally` 也必须在:正常走完时收尾。

两条路都会跑到,于是 `return()` 被调两次。对持有订阅的 async iterator
来说那是真实的 double-close。

    let finalised = false;
    const finalise = async () => {
      if (finalised) return;
      finalised = true;
      await iterator.return?.();
    };

局部 `let` 就够:listener 和 `finally` 共享这个闭包和这次调用。

## 测试改用排空微任务

`finally` 不在时钟上,它在 parked pull resolve 之后的若干次微任务跳:
abort listener → `await iterator.next()` 延续 → break → finally。跳数是确定
的,时钟不是。所以数微任务,再补一次 setImmediate(socket close 到 abort
listener 走的是宏任务队列)。

这条测试守的就是这个修复,所以它必须在不靠睡眠的情况下成立。

Author: probe

packages/webui/src/server/operation/operation-dispatch.ts

@@ -93,7 +93,25 @@ export async function dispatchWebuiFrame(
           result.stream.source as AsyncIterable<unknown> | Iterable<unknown>,
         );
         const signal = getSignal();
-        const close = () => void iterator.return?.();
+        // One finalisation, two routes to reach it.
+        //
+        // The abort listener exists for the case that made this non-obvious: a
+        // client that disconnects mid-stream leaves the pump parked on a `next()`
+        // that may never settle, so the `finally` below would never run. The
+        // listener is the only thing that can finish that iterator.
+        //
+        // But when the pull DOES settle, the loop breaks and the `finally` runs
+        // too — and calling `return()` twice on a live async iterator is a real
+        // double-close on whatever subscription it holds. Hence one idempotent
+        // gate rather than two call sites. A local `let` is enough: the
+        // listener and the `finally` share this closure and this invocation.
+        let finalised = false;
+        const finalise = async (): Promise<void> => {
+          if (finalised) return;
+          finalised = true;
+          await iterator.return?.();
+        };
+        const close = () => void finalise();
         signal?.addEventListener("abort", close, { once: true });
         // Acknowledge before pumping, but only for the operations that opt
         // in. The data streams keep their original wire shape so any
@@ -111,7 +129,7 @@ export async function dispatchWebuiFrame(
           }
         } finally {
           signal?.removeEventListener("abort", close);
-          await iterator.return?.();
+          await finalise();
         }
         return;
       }