fix(acp): restore IteratorResult return type to nextSdkMessageOrAbort

The simplified SDKMessage | undefined return type collapsed two distinct
states: generator truly done vs generator yielding undefined. This broke
forwardSessionUpdates which needs to distinguish the two — when the
generator yields null/undefined it should continue (calling next() again),
not break out of the loop.

Restored the original IteratorResult<SDKMessage, void> return type so
done and yielded-null are distinct again.

src/services/acp/__tests__/bridge.test.ts

@@ -698,7 +698,7 @@ describe('toDisplayPath', () => {
 // ── forwardSessionUpdates ─────────────────────────────────────────
 
 describe('nextSdkMessageOrAbort', () => {
-  test('returns undefined when aborted while waiting for next message', async () => {
+  test('returns done:true when aborted while waiting for next message', async () => {
     const ac = new AbortController()
     const pending = nextSdkMessageOrAbort(makeWaitingStream(), ac.signal)
     ac.abort()
@@ -708,19 +708,19 @@ describe('nextSdkMessageOrAbort', () => {
       new Promise<'timeout'>(resolve => setTimeout(resolve, 100, 'timeout')),
     ])
 
-    expect(result).toBeUndefined()
+    expect(result).toEqual({ done: true, value: undefined })
   })
 
-  test('returns undefined when stream is done', async () => {
+  test('returns done:true when stream is done', async () => {
     const result = await nextSdkMessageOrAbort(
       makeStream([]),
       new AbortController().signal,
     )
 
-    expect(result).toBeUndefined()
+    expect(result).toEqual({ done: true, value: undefined })
   })
 
-  test('returns a valid SDKMessage', async () => {
+  test('returns a valid SDKMessage via IteratorResult', async () => {
     const msg = {
       type: 'assistant',
       message: {
@@ -734,7 +734,7 @@ describe('nextSdkMessageOrAbort', () => {
       new AbortController().signal,
     )
 
-    expect(result).toBe(msg)
+    expect(result).toEqual({ done: false, value: msg })
   })
 })
 

src/services/acp/bridge.ts

@@ -719,28 +719,22 @@ export function toolUpdateFromEditToolResponse(toolResponse: unknown): {
 export function nextSdkMessageOrAbort(
   sdkMessages: AsyncGenerator<SDKMessage, void, unknown>,
   abortSignal: AbortSignal,
-): Promise<SDKMessage | undefined> {
+): Promise<IteratorResult<SDKMessage, void>> {
   if (abortSignal.aborted) {
-    return Promise.resolve(undefined)
+    return Promise.resolve({ done: true, value: undefined })
   }
   let abortHandler: (() => void) | undefined
-  const abortPromise = new Promise<undefined>(resolve => {
-    abortHandler = () => resolve(undefined)
-    abortSignal.addEventListener('abort', abortHandler, { once: true })
+  const abortPromise = new Promise<IteratorResult<SDKMessage, void>>(
+    resolve => {
+      abortHandler = () => resolve({ done: true, value: undefined })
+      abortSignal.addEventListener('abort', abortHandler, { once: true })
+    },
+  )
+  return Promise.race([sdkMessages.next(), abortPromise]).finally(() => {
+    if (abortHandler) {
+      abortSignal.removeEventListener('abort', abortHandler)
+    }
   })
-  const nextPromise = sdkMessages.next()
-  return Promise.race([nextPromise, abortPromise])
-    .then(result => {
-      if (result === undefined) {
-        void nextPromise.catch(() => {})
-        return undefined
-      }
-      if (result.done === true) return undefined
-      return result.value
-    })
-    .finally(() => {
-      if (abortHandler) abortSignal.removeEventListener('abort', abortHandler)
-    })
 }
 
 // ── Main forwarding function ──────────────────────────────────────
@@ -779,8 +773,10 @@ export async function forwardSessionUpdates(
       // Race the next message against the abort signal so we unblock
       // immediately when cancelled, even if the generator is waiting for
       // a slow API response.
-      const rawMsg = await nextSdkMessageOrAbort(sdkMessages, abortSignal)
-      if (rawMsg === undefined || abortSignal.aborted) break
+      const nextResult = await nextSdkMessageOrAbort(sdkMessages, abortSignal)
+      if (nextResult.done || abortSignal.aborted) break
+      const rawMsg = nextResult.value
+      if (rawMsg == null) continue
       const msg = rawMsg as BridgeSDKMessage
 
       switch (msg.type) {