Tighten binstub sync coverage and helper specs (#1123)

Address the medium-priority test-coverage follow-ups from the PR #1104
review.

1. Cover spec/dummy/bin/shakapacker-config in the sync spec. The dummy
   binstub used by the test app must stay byte-identical to the install
   template; otherwise a stale wrapper could silently mask install-
   template changes.

2. Add a JS test that invokes createBinStub for both helper binstub
   names and asserts the generated content is byte-identical to the
   checked-in lib/install/bin/* files. Exports createBinStub from
   package/configExporter/cli.ts so the test can reach it. This closes
   the gap where the Ruby sync spec couldn't reach into the JS template.

3. Assert the `[Shakapacker] No Gemfile found at …` warning in
   helper_binstubs_spec.rb so the fallback path can't lose its warning
   silently.

4. Update the "Keep in sync" comment in all four locations to list all
   four copies (the previous wording said "all three" and missed the
   dummy binstub).

5. Rename the "documents every divergent binstub and only divergent
   binstubs" example to "all documented divergent binstubs still exist
   in both directories" so the name matches what the test actually
   verifies — the "only" guarantee comes from the iteration loop, not
   this example.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: justin808

package/configExporter/cli.ts

@@ -1,7 +1,13 @@
 // This will be a substantial file - the main CLI entry point
 // Originally migrated from bin/export-bundler-config, now bin/shakapacker-config
 
-import { existsSync, readFileSync, writeFileSync } from "fs"
+import {
+  chmodSync,
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  writeFileSync
+} from "fs"
 import { resolve, dirname, sep, delimiter, basename } from "path"
 import { inspect } from "util"
 import { load as loadYaml } from "js-yaml"
@@ -510,10 +516,10 @@ function runInitCommand(options: ExportOptions): number {
   return 0
 }
 
-function createBinStub(binStubPath: string): void {
+// Exported for test use only: verifies generated content matches lib/install/bin/* binstubs.
+export function createBinStub(binStubPath: string): void {
   const binDir = dirname(binStubPath)
   const packageScript = `${basename(binStubPath)}.cjs`
-  const { mkdirSync, chmodSync } = require("fs")
 
   // Ensure bin directory exists
   if (!existsSync(binDir)) {

test/configExporter/createBinStub.test.js

@@ -0,0 +1,44 @@
+const { mkdtempSync, readFileSync, rmSync } = require("fs")
+const { join, resolve } = require("path")
+const { tmpdir } = require("os")
+const { createBinStub } = require("../../package/configExporter/cli")
+
+const gemRoot = resolve(__dirname, "../..")
+
+// The Ruby logic in lib/install/bin/shakapacker-config and
+// lib/install/bin/diff-bundler-config is also duplicated inside the
+// `createBinStub` template in package/configExporter/cli.ts. The Ruby spec
+// (spec/shakapacker/binstub_sync_spec.rb) keeps the three checked-in copies
+// (install template, install diff template, and the dummy app's binstub)
+// honest, but it cannot reach into the JS template. This test closes that
+// gap by invoking createBinStub for both helper names and asserting the
+// generated content matches the corresponding lib/install/bin/* file.
+describe("createBinStub template parity", () => {
+  let tmp
+
+  beforeEach(() => {
+    tmp = mkdtempSync(join(tmpdir(), "shakapacker-createBinStub-"))
+  })
+
+  afterEach(() => {
+    if (tmp) {
+      rmSync(tmp, { recursive: true, force: true })
+    }
+  })
+
+  test.each([["shakapacker-config"], ["diff-bundler-config"]])(
+    "generates lib/install/bin/%s byte-for-byte",
+    (binstubName) => {
+      const generatedPath = join(tmp, "bin", binstubName)
+      createBinStub(generatedPath)
+
+      const generated = readFileSync(generatedPath, "utf8")
+      const installed = readFileSync(
+        join(gemRoot, "lib", "install", "bin", binstubName),
+        "utf8"
+      )
+
+      expect(generated).toBe(installed)
+    }
+  )
+})