fix: improve exec stream tests cross-platform handling

- Update spawn-wait-for-file.js to use proper stdio inheritance
- Add small delay before exit to ensure child process inherits handles
- Simplify test code to use the helper script instead of shell commands

packages/exec/__tests__/scripts/spawn-wait-for-file.js

@@ -1,51 +1,39 @@
 const childProcess = require('child_process')
 const path = require('path')
 
-function parseArgs() {
-  const result = {}
-  for (const arg of process.argv.slice(2)) {
-    const equalsIndex = arg.indexOf('=')
-    if (equalsIndex === -1) {
-      continue
-    }
-    const key = arg.slice(0, equalsIndex)
-    const value = arg.slice(equalsIndex + 1)
-    result[key] = value
+// Parse args
+const args = {}
+for (const arg of process.argv.slice(2)) {
+  const idx = arg.indexOf('=')
+  if (idx !== -1) {
+    args[arg.slice(0, idx)] = arg.slice(idx + 1)
   }
-
-  return result
 }
 
-const args = parseArgs()
 const filePath = args.file
 if (!filePath) {
   throw new Error('file is not specified')
 }
 
+// Spawn wait-for-file.js with inherited stdio
+// This creates a grandchild process that holds the stdio handles open
+// after this process (the child) exits
 const waitScript = path.join(__dirname, 'wait-for-file.js')
-const waitArgs = [waitScript, `file=${filePath}`]
+const child = childProcess.spawn(process.execPath, [waitScript, `file=${filePath}`], {
+  stdio: ['ignore', 'inherit', 'inherit'],
+  detached: process.platform !== 'win32'
+})
 
-// Spawn with inherited stdio and detached on Unix, non-detached on Windows
-// This keeps the streams open after parent exits
-const isWindows = process.platform === 'win32'
-const spawnOptions = {
-  stdio: 'inherit',
-  detached: !isWindows
-}
-
-// On Windows, we need to hide the window
-if (isWindows) {
-  spawnOptions.windowsHide = true
-}
-
-const waitProcess = childProcess.spawn(process.execPath, waitArgs, spawnOptions)
-
-// Unref so parent doesn't wait for child
-waitProcess.unref()
+// Don't wait for child to exit
+child.unref()
 
+// Handle optional stderr output (must happen BEFORE we exit)
 if (args.stderr === 'true') {
   process.stderr.write('hi')
 }
 
-const exitCode = args.exitCode ? parseInt(args.exitCode, 10) : 0
-process.exit(exitCode)
+// Small delay to ensure child has started and inherited handles
+setTimeout(() => {
+  const exitCode = args.exitCode ? parseInt(args.exitCode, 10) : 0
+  process.exit(exitCode)
+}, 50)