test(pack): cover the missing lightningcss optional peer error

Add the command-pack-css-missing-lightningcss snap test: a resolve hook
makes lightningcss unresolvable (the dev/CI monorepo always has it), so
vp pack surfaces the actionable install hint. Locks in that the error
tells users how to fix it.

Author: fengmk2

packages/cli/snap-tests/command-pack-css-missing-lightningcss/block-lightningcss.mjs

@@ -0,0 +1,11 @@
+// Simulate `lightningcss` not being installed: the dev/CI monorepo always has it
+// resolvable from core (it is a transitive dependency), so a resolve hook is the
+// deterministic way to exercise the missing optional-peer path. See issue #1586.
+export async function resolve(specifier, context, nextResolve) {
+  if (specifier === 'lightningcss') {
+    const error = new Error("Cannot find package 'lightningcss'");
+    error.code = 'ERR_MODULE_NOT_FOUND';
+    throw error;
+  }
+  return nextResolve(specifier, context);
+}

packages/cli/snap-tests/command-pack-css-missing-lightningcss/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "command-pack-css-missing-lightningcss",
+  "version": "1.0.0",
+  "type": "module"
+}

packages/cli/snap-tests/command-pack-css-missing-lightningcss/register-block.mjs

@@ -0,0 +1,5 @@
+import { register } from 'node:module';
+
+// Loaded via `NODE_OPTIONS=--import=./register-block.mjs` so the resolve hook
+// applies to the `vp pack` process (and its managed-Node child).
+register('./block-lightningcss.mjs', import.meta.url);

packages/cli/snap-tests/command-pack-css-missing-lightningcss/snap.txt

@@ -0,0 +1,9 @@
+[1]> NODE_OPTIONS=--import=./register-block.mjs vp pack src/index.ts --minify # missing optional lightningcss peer prints an actionable install hint (issue #1586)
+ℹ entry: src/index.ts
+ℹ Build start
+error: Build failed with 1 error:
+
+[plugin @tsdown/css] <cwd>/src/style.css
+Error: Cannot find package "lightningcss". CSS bundling with `vp pack` requires it. Install it with `vp add -D lightningcss`.
+Caused by:
+  Error: Cannot find package 'lightningcss'

packages/cli/snap-tests/command-pack-css-missing-lightningcss/src/index.ts

@@ -0,0 +1,3 @@
+import './style.css';
+
+export const hello = 'world';

packages/cli/snap-tests/command-pack-css-missing-lightningcss/src/style.css

@@ -0,0 +1,3 @@
+.foo {
+  color: #ff0000;
+}

packages/cli/snap-tests/command-pack-css-missing-lightningcss/steps.json

@@ -0,0 +1,6 @@
+{
+  "ignoredPlatforms": ["win32"],
+  "commands": [
+    "NODE_OPTIONS=--import=./register-block.mjs vp pack src/index.ts --minify # missing optional lightningcss peer prints an actionable install hint (issue #1586)"
+  ]
+}

rfcs/pack-command.md

@@ -389,6 +389,16 @@ Tests `vp pack src/index.ts --minify` on a CSS entry: the bundled `@tsdown/css`
 
 Loads the bundled `dist/tsdown/tsdown-exe.js` and `dist/tsdown/tsdown-css.js` chunks to prove they resolve `tsdown/internal` against the bundled tsdown without a top-level `tsdown` package. This is Node-version independent (it does not run the SEA build), so it catches the import-resolution regression on every CI Node version.
 
+### Local CLI Test: `command-pack-css-missing-lightningcss`
+
+**Location**: `packages/cli/snap-tests/command-pack-css-missing-lightningcss/`
+
+Runs `vp pack --minify` with a resolve hook (`NODE_OPTIONS=--import`) that makes `lightningcss` unresolvable, simulating the uninstalled optional peer (the dev/CI monorepo always has it). Asserts the actionable error tells the user how to fix it:
+
+```
+Cannot find package "lightningcss". CSS bundling with `vp pack` requires it. Install it with `vp add -D lightningcss`.
+```
+
 ### Global CLI Test: `command-pack-exe`
 
 **Location**: `packages/cli/snap-tests-global/command-pack-exe/`