test(env): add snap test for vp env off/on mode switching

Verifies the full system-first mode lifecycle:
- `vp run` uses project's engines.node in managed mode
- `vp env off` switches to system-first mode
- `vp run` uses system Node.js instead of project version
- `vp env on` restores managed mode

Author: fengmk2

packages/cli/snap-tests-global/command-env-off-on/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "command-env-off-on",
+  "version": "1.0.0",
+  "private": true,
+  "engines": {
+    "node": "20.18.0"
+  },
+  "scripts": {
+    "check-node": "node -e \"console.log(process.version)\""
+  }
+}

packages/cli/snap-tests-global/command-env-off-on/snap.txt

@@ -0,0 +1,38 @@
+> vp run check-node # Managed mode: should use project's engines.node (20.18.0)
+VITE+ - The Unified Toolchain for the Web
+
+$ node -e "console.log(process.version)" ⊘ cache disabled
+v<semver>
+
+
+> vp env off # Switch to system-first mode
+VITE+ - The Unified Toolchain for the Web
+
+✓ Node.js management set to system-first.
+
+All vp commands and shims will now prefer system Node.js, falling back to managed if not found.
+
+Run `vp env on` to always use Vite+ managed Node.js.
+
+> vp run check-node # System-first mode: should use system Node.js, not project's 20.18.0
+VITE+ - The Unified Toolchain for the Web
+
+$ node -e "console.log(process.version)" ⊘ cache disabled
+v<semver>
+
+
+> vp env on # Switch back to managed mode
+VITE+ - The Unified Toolchain for the Web
+
+✓ Node.js management set to managed.
+
+All vp commands and shims will now always use Vite+ managed Node.js.
+
+Run `vp env off` to prefer system Node.js instead.
+
+> vp run check-node # Managed mode restored: should use project's engines.node again
+VITE+ - The Unified Toolchain for the Web
+
+$ node -e "console.log(process.version)" ⊘ cache disabled
+v<semver>
+

packages/cli/snap-tests-global/command-env-off-on/steps.json

@@ -0,0 +1,12 @@
+{
+  "serial": true,
+  "ignoredPlatforms": ["win32"],
+  "commands": [
+    "vp run check-node # Managed mode: should use project's engines.node (20.18.0)",
+    "vp env off # Switch to system-first mode",
+    "vp run check-node # System-first mode: should use system Node.js, not project's 20.18.0",
+    "vp env on # Switch back to managed mode",
+    "vp run check-node # Managed mode restored: should use project's engines.node again"
+  ],
+  "after": ["vp env on"]
+}