Fix neverBundle callback signature to match tsdown v0.22.0 API

The neverBundle callback's second parameter should be `parentId?: string`
(optional) per tsdown/rolldown's ExternalOptionFunction type, not
`importer: string | undefined` (required). While TypeScript's callback
compatibility allows passing fewer parameters, aligning the name and
optionality with the documented API makes the intent clearer and avoids
any future type mismatch if the type is checked strictly.

#766 (comment)

Assisted-by: Claude Code:claude-sonnet-4-6

Author: dahlia

packages/fedify/tsdown.config.ts

@@ -46,10 +46,10 @@ export default [
         .map((f) => f.replace(sep, "/")),
     ],
     deps: {
-      neverBundle: (id: string, importer: string | undefined) => {
+      neverBundle: (id: string, parentId?: string) => {
         if (id.startsWith("node:")) return true;
         if (id !== "@fedify/fixture") return;
-        return !isTestingHelperImporter(importer);
+        return !isTestingHelperImporter(parentId);
       },
       // Bundle @fedify/fixture back in for src/testing/ files (needed for
       // cfworkers), while keeping it external for test files so that