fix(build): resolve synthesized not-found entries on windows

The bundler hands entries over as native paths, which never matched the
posix `srcDir`, so a synthesized `<locale>/404.md` was left unresolved on
windows. Ids are normalized before matching, with drive letters compared
case-insensitively. The plugin's unit tests use an `.mjs` config like the
other node tests, which keeps vite's config loader quiet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

__tests__/unit/node/plugins/notFoundPlugin.test.ts

@@ -12,7 +12,7 @@ async function site(files: Record<string, string>, config = '') {
   const root = mkdtempSync(join(tmpdir(), 'vp-not-found-'))
   mkdirSync(join(root, '.vitepress'), { recursive: true })
   writeFileSync(
-    join(root, '.vitepress/config.ts'),
+    join(root, '.vitepress/config.mjs'),
     `export default { ${locales}, ${config} }`
   )
   for (const [file, content] of Object.entries(files)) {
@@ -22,8 +22,8 @@ async function site(files: Record<string, string>, config = '') {
   const siteConfig = await resolveConfig(root, 'build', 'production')
   const plugin = notFoundPlugin(siteConfig)
   const hooks = {
-    resolveId: (id: string) =>
-      (plugin.resolveId as any).handler.call(undefined, id, undefined, {}),
+    resolveId: (id: string, importer?: string) =>
+      (plugin.resolveId as any).handler.call(undefined, id, importer, {}),
     load: (id: string) => (plugin.load as any).handler.call(undefined, id)
   }
   const file = (page: string) => normalizePath(join(siteConfig.srcDir, page))
@@ -45,6 +45,13 @@ describe('node/plugins/notFoundPlugin', () => {
       ])
       expect(s.resolveId('/zh/404.md')).toBe(s.file('zh/404.md'))
       expect(s.resolveId('/zh/404.md?t=123')).toBe(s.file('zh/404.md'))
+      // the bundler hands entries over as native paths
+      expect(s.resolveId(join(s.siteConfig.srcDir, 'zh', '404.md'))).toBe(
+        s.file('zh/404.md')
+      )
+      expect(s.resolveId('./zh/404.md', s.file('index.md'))).toBe(
+        s.file('zh/404.md')
+      )
       expect(s.load(s.file('404.md'))).toContain('<NotFound />')
       expect(s.load(s.file('zh/404.md'))).toContain('<NotFound />')
     } finally {

src/node/plugins/notFoundPlugin.ts

@@ -42,6 +42,13 @@ export const notFoundPlugin = (siteConfig: SiteConfig): Plugin => {
     return index === -1 ? [id, ''] : [id.slice(0, index), id.slice(index + 1)]
   }
 
+  // ids arrive as urls, posix paths or native windows paths (the bundler
+  // entries), with the drive letter in either case
+  const isUnderSrcDir = (file: string) =>
+    file
+      .replace(/^[a-z]:/i, (d) => d.toLowerCase())
+      .startsWith(srcDir.replace(/^[a-z]:/i, (d) => d.toLowerCase()))
+
   // the synthesized page a would-be file stands for, and the authored root
   // page it inherits when there is one
   const virtualPage = (file: string) => {
@@ -64,15 +71,17 @@ export const notFoundPlugin = (siteConfig: SiteConfig): Plugin => {
     resolveId: {
       filter: { id: notFoundRE },
       handler(id, importer) {
-        const [file, query] = splitQuery(id)
+        const [rawFile, query] = splitQuery(id)
         // sub-requests (`?vue&type=…`) belong to the module that owns them
         if (query && !/^t=\d+$/.test(query)) return
-        const resolved = file.startsWith(srcDir)
+        // normalizing first would fold `./x` into `x`, so test the raw id
+        const file = normalizePath(rawFile)
+        const resolved = isUnderSrcDir(file)
           ? file
-          : file.startsWith('/')
-            ? normalizePath(path.join(srcDir, file))
-            : importer && file.startsWith('.')
-              ? normalizePath(path.resolve(path.dirname(importer), file))
+          : rawFile.startsWith('/')
+            ? normalizePath(path.join(srcDir, rawFile))
+            : importer && rawFile.startsWith('.')
+              ? normalizePath(path.resolve(path.dirname(importer), rawFile))
               : undefined
         const page = resolved && virtualPage(resolved)
         if (page) return page.inherits ? VIRTUAL_PREFIX + resolved : resolved