Use noExternal to selectively bundle @fedify/fixture for testing/mod.ts

The previous approach of splitting src/testing/mod.ts into a separate
defineConfig caused the cfworkers build to fail because tsdown flattened
the single-entry output to dist/mod.js instead of dist/testing/mod.js,
breaking the import in src/cfworkers/server.ts.

Instead, keep src/testing/mod.ts and *.test.ts in a single defineConfig
(preserving directory structure in output), mark @fedify/fixture as
external, and use noExternal with a function to selectively bundle it
back in only when imported from src/testing/ files.  This way:

 -  The cfworkers build gets @fedify/fixture inlined in testing/mod.js
 -  Test files keep @fedify/fixture external, avoiding the
    UNLOADABLE_DEPENDENCY error during pnpm pack --recursive

https://github.com/fedify-dev/fedify/actions/runs/23243929108/job/67567201018

Co-authored-by: Amp (https://ampcode.com)
Amp-Thread-ID: https://ampcode.com/threads/T-019d00c0-4bd0-70fe-b81f-82e3738959e9
Co-authored-by: Amp <amp@ampcode.com>

Author: dahlia

packages/fedify/tsdown.config.ts

@@ -35,8 +35,20 @@ export default [
     },
   }),
   defineConfig({
-    entry: ["./src/testing/mod.ts"],
-    external: [/^node:/],
+    entry: [
+      "./src/testing/mod.ts",
+      ...(await Array.fromAsync(glob(`src/**/*.test.ts`)))
+        .map((f) => f.replace(sep, "/")),
+    ],
+    external: [/^node:/, "@fedify/fixture"],
+    // Bundle @fedify/fixture back in for src/testing/ files (needed for
+    // cfworkers), while keeping it external for test files so that
+    // pnpm pack --recursive does not try to resolve the private package:
+    noExternal: (id: string, importer: string | undefined) => {
+      if (id !== "@fedify/fixture") return false;
+      const normalized = importer?.replaceAll(sep, "/");
+      return normalized?.includes("/testing/") ?? false;
+    },
     inputOptions: {
       onwarn(warning, defaultHandler) {
         if (
@@ -57,18 +69,6 @@ export default [
     `,
     },
   }),
-  defineConfig({
-    entry: (await Array.fromAsync(glob(`src/**/*.test.ts`)))
-      .map((f) => f.replace(sep, "/")),
-    external: [/^node:/, "@fedify/fixture"],
-    outputOptions: {
-      intro: `
-      import { Temporal } from "@js-temporal/polyfill";
-      import { URLPattern } from "urlpattern-polyfill";
-      globalThis.addEventListener = () => {};
-    `,
-    },
-  }),
 ];
 
 // cSpell: ignore onwarn