test(acp-snapshot): detach the inherited-stdio fixture

Windows does not guarantee that an ordinary child process will continue after its parent exits. The fake ACP agent exited immediately after spawning its late-output descendant, so Windows could tear down that descendant and close the protocol stream before the delayed ACP update was written.

Launch the descendant in detached mode while continuing to inherit stdout and stderr, then unref it as before. This preserves the intended regression boundary: launcher shutdown must wait for descendant-held stdio and parse the final buffered frame after the direct ACP parent exits.

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

@@ -267,6 +267,7 @@ function flushLogsAndExit(): void {
       `setTimeout(() => process.stderr.write(${JSON.stringify('late inherited stderr\n')}), 75)`,
     ].join(';')
     spawn(process.execPath, ['-e', code], {
+      detached: true,
       stdio: ['ignore', 'inherit', 'inherit'],
     }).unref()
   }