fix(tests): preserve process.env proxy for Windows

The spread operator {...process.env} breaks Windows environment variable
handling because it loses the special proxy behavior that Node.js uses
for process.env. On Windows, environment variables are case-insensitive
and accessed through a proxy that the spread operator doesn't preserve.

This was causing tests to produce empty CLI output on Windows runners
because critical environment variables weren't properly passed to
spawned processes.

Changes:
- Set VITEST='1' directly on process.env instead of spreading
- Restore processEnv to direct process.env reference
- Add comments explaining Windows compatibility requirement

This reverts processEnv behavior to match commit 39ee946 which
had working Windows tests.

Fixes: Empty CLI output in Windows CI tests (504 test failures)

Author: jdalton

packages/cli/test/utils.mts

@@ -16,14 +16,18 @@ import {
 const __filename = fileURLToPath(import.meta.url)
 const __dirname = path.dirname(__filename)
 
+// Set VITEST environment variable for test runs.
+// This disables interactive help menus in spawned CLI processes.
+// Must be set on process.env directly (not spread) to preserve
+// Windows environment variable proxy behavior.
+if (!process.env['VITEST']) {
+  process.env['VITEST'] = '1'
+}
+
 // Backward compatibility object for tests.
 const constants = {
   execPath,
-  processEnv: {
-    ...process.env,
-    // Disable interactive help menu in spawned CLI processes during tests.
-    VITEST: '1',
-  },
+  processEnv: process.env,
 }
 
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