fix: resolve npm for portable runtime packaging

tests/npm-invocation.test.ts

@@ -0,0 +1,60 @@
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { createRequire } from 'node:module'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+
+interface NpmInvocationModule {
+  npmInvocation(
+    projectDir: string,
+    environment?: NodeJS.ProcessEnv
+  ): {
+    command: string
+    prefixArgs: string[]
+  }
+}
+
+const require = createRequire(import.meta.url)
+const { npmInvocation } = require(
+  '../build/npm-invocation.cjs'
+) as NpmInvocationModule
+
+let directory: string
+
+beforeEach(() => {
+  directory = mkdtempSync(join(tmpdir(), 'goodbuddy-npm-invocation-'))
+})
+
+afterEach(() => {
+  rmSync(directory, { recursive: true, force: true })
+})
+
+describe('npm invocation', () => {
+  it('uses npm_execpath when the package manager provides it', () => {
+    expect(
+      npmInvocation(directory, { npm_execpath: 'provided-npm-cli.js' })
+    ).toEqual({
+      command: process.execPath,
+      prefixArgs: ['provided-npm-cli.js']
+    })
+  })
+
+  it('uses the project npm CLI when npm_execpath is absent', () => {
+    const npmCli = join(
+      directory,
+      'node_modules',
+      'npm',
+      'bin',
+      'npm-cli.js'
+    )
+    mkdirSync(join(directory, 'node_modules', 'npm', 'bin'), {
+      recursive: true
+    })
+    writeFileSync(npmCli, '')
+
+    expect(npmInvocation(directory, {})).toEqual({
+      command: process.execPath,
+      prefixArgs: [npmCli]
+    })
+  })
+})