test(acp-snapshot): inherit descendant stdio portably

Pass the fake descendant the parent process's stdout and stderr stream handles instead of Unix-style numeric file descriptors. This lets Windows duplicate the live ACP and diagnostic pipes so launcher shutdown can prove that inherited handles, buffered frames, and stderr all drain after the parent exits.

Observe the pending update promise before initiating shutdown as well, preventing a missing late frame from becoming a transient unhandled rejection before the assertion reports the fixture failure.

packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts

@@ -266,7 +266,9 @@ function flushLogsAndExit(): void {
       `setTimeout(() => process.stdout.write(${JSON.stringify(`${frame}\n`)}), 50)`,
       `setTimeout(() => process.stderr.write(${JSON.stringify('late inherited stderr\n')}), 75)`,
     ].join(';')
-    spawn(process.execPath, ['-e', code], { stdio: ['ignore', 1, 2] }).unref()
+    spawn(process.execPath, ['-e', code], {
+      stdio: ['ignore', process.stdout, process.stderr],
+    }).unref()
   }
   process.exit(0)
 }

packages/support/acp-snapshot/tests/harness.spec.ts

@@ -139,6 +139,9 @@ describe('runScenario', () => {
       update.sessionUpdate === 'agent_message_chunk'
       && update.content.type === 'text'
       && update.content.text === 'late inherited stdout')
+    // Arm rejection handling before close may exhaust the stream; the later assertion still
+    // observes the original promise and turns a missing inherited frame into the test failure.
+    void lateUpdate.catch(() => undefined)
 
     await launched.close()