Fix npm-spec plugin installs when npm pack output is empty (#21039)

* fix(plugins): recover npm pack archive when stdout is empty

* test(plugins): create npm pack archive in metadata mock

---------

Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/test-utils/npm-spec-install-test-helpers.ts

@@ -1,5 +1,7 @@
+import fs from "node:fs";
+import path from "node:path";
 import { expect } from "vitest";
-import type { SpawnResult } from "../process/exec.js";
+import type { CommandOptions, SpawnResult } from "../process/exec.js";
 import { expectSingleNpmInstallIgnoreScriptsCall } from "./exec-assertions.js";
 
 export type InstallResultLike = {
@@ -40,10 +42,31 @@ export async function expectUnsupportedNpmSpec(
 }
 
 export function mockNpmPackMetadataResult(
-  run: { mockResolvedValue: (value: SpawnResult) => unknown },
+  run: {
+    mockImplementation: (
+      implementation: (
+        argv: string[],
+        optionsOrTimeout: number | CommandOptions,
+      ) => Promise<SpawnResult>,
+    ) => unknown;
+  },
   metadata: NpmPackMetadata,
 ) {
-  run.mockResolvedValue(createSuccessfulSpawnResult(JSON.stringify([metadata])));
+  run.mockImplementation(async (argv, optionsOrTimeout) => {
+    if (argv[0] !== "npm" || argv[1] !== "pack") {
+      throw new Error(`unexpected command: ${argv.join(" ")}`);
+    }
+
+    const cwd =
+      typeof optionsOrTimeout === "object" && optionsOrTimeout !== null
+        ? optionsOrTimeout.cwd
+        : undefined;
+    if (cwd) {
+      fs.writeFileSync(path.join(cwd, metadata.filename), "");
+    }
+
+    return createSuccessfulSpawnResult(JSON.stringify([metadata]));
+  });
 }
 
 export function expectIntegrityDriftRejected(params: {