Remove regenerateBundlerBinStub workaround for bundler 2.7+

Bundler 2.7+ philosophy: 'Bundler itself does not use binstubs because
its version is selected by RubyGems'

The regenerateBundlerBinStub() function was added in 2017 (commit fe6e899)
to work around bundler 1.16.0 incompatibility with Rails-generated binstubs.

This workaround is no longer necessary because:
1. The original Rails binstub bug was fixed years ago
2. Bundler 2.7+ explicitly refuses to create its own binstub by design
3. The buildpack doesn't use deps/0/bin/bundle anyway - it uses the system
   bundle command from deps/0/bundler (configured via GEM_PATH)
4. Modern bundler philosophy is to use the system bundle, not a binstub

Changes:
- Removed regenerateBundlerBinStub() function
- Removed call to regenerateBundlerBinStub() from InstallGems()
- Removed related unit tests (setupBundlerBin, itRegeneratesBundleBinstub)
- All 76 unit tests pass

This aligns the buildpack with bundler 2.7+ architectural decisions.

Author: ramonskie

src/ruby/supply/supply.go

@@ -768,10 +768,6 @@ func (s *Supplier) InstallGems() error {
 		return err
 	}
 
-	if err := s.regenerateBundlerBinStub(tempDir); err != nil {
-		return err
-	}
-
 	s.Log.Info("Cleaning up the bundler cache.")
 	cmd = exec.Command("bundle", "clean")
 	cmd.Dir = tempDir
@@ -845,18 +841,6 @@ func (s *Supplier) removeIncompatibleBundledWithVersion(bundledWithVersion strin
 	return os.WriteFile(gemfileLockPath, output, 0666)
 }
 
-func (s *Supplier) regenerateBundlerBinStub(appDir string) error {
-	s.Log.BeginStep("Regenerating bundler binstubs...")
-	cmd := exec.Command("bundle", "binstubs", "bundler", "--force", "--path", filepath.Join(s.Stager.DepDir(), "binstubs"))
-	cmd.Dir = appDir
-	cmd.Stdout = text.NewIndentWriter(os.Stdout, []byte("       "))
-	cmd.Stderr = text.NewIndentWriter(os.Stderr, []byte("       "))
-	if err := s.Command.Run(cmd); err != nil {
-		return err
-	}
-	return libbuildpack.CopyFile(filepath.Join(s.Stager.DepDir(), "binstubs", "bundle"), filepath.Join(s.Stager.DepDir(), "bin", "bundle"))
-}
-
 func (s *Supplier) EnableLDLibraryPathEnv() error {
 	if exists, err := libbuildpack.FileExists(filepath.Join(s.Stager.BuildDir(), "ld_library_path")); err != nil {
 		return err

src/ruby/supply/supply_test.go

@@ -8,8 +8,6 @@ import (
 	"os/exec"
 	"path/filepath"
 
-	"reflect"
-
 	"github.com/cloudfoundry/ruby-buildpack/src/ruby/cache"
 	"github.com/cloudfoundry/ruby-buildpack/src/ruby/supply"
 
@@ -481,32 +479,13 @@ var _ = Describe("Supply", func() {
 
 		const windowsWarning = "**WARNING** Windows line endings detected in Gemfile. Your app may fail to stage. Please use UNIX line endings."
 
-		handleBundleBinstubRegeneration := func(cmd *exec.Cmd) error {
-			if len(cmd.Args) > 5 && reflect.DeepEqual(cmd.Args[0:5], []string{"bundle", "binstubs", "bundler", "--force", "--path"}) {
-				Expect(cmd.Args[5]).To(HavePrefix(filepath.Join(depsDir, depsIdx)))
-				Expect(os.MkdirAll(cmd.Args[5], 0755)).To(Succeed())
-				Expect(os.WriteFile(filepath.Join(cmd.Args[5], "bundle"), []byte("new bundle binstub"), 0644)).To(Succeed())
-			}
-			return nil
-		}
-
-		itRegeneratesBundleBinstub := func() {
-			It("Re-generates the bundler binstub to replace older, rails-generated ones that are incompatible with bundler > 1.16.0", func() {
-				Expect(supplier.InstallGems()).To(Succeed())
-				Expect(os.ReadFile(filepath.Join(depsDir, depsIdx, "binstubs", "bundle"))).To(Equal([]byte("new bundle binstub")))
-				Expect(os.ReadFile(filepath.Join(depsDir, depsIdx, "bin", "bundle"))).To(Equal([]byte("new bundle binstub")))
-			})
-		}
-
 		Context("Windows Gemfile", func() {
 			BeforeEach(func() {
 				mockVersions.EXPECT().HasWindowsGemfileLock().Return(false, nil)
-				mockCommand.EXPECT().Run(gomock.Any()).AnyTimes().Do(handleBundleBinstubRegeneration)
+				mockCommand.EXPECT().Run(gomock.Any()).AnyTimes()
 				Expect(os.WriteFile(filepath.Join(buildDir, "Gemfile"), []byte("source \"https://rubygems.org\"\r\ngem \"rack\"\r\n"), 0644)).To(Succeed())
 			})
 
-			itRegeneratesBundleBinstub()
-
 			It("Warns the user", func() {
 				Expect(supplier.InstallGems()).To(Succeed())
 				Expect(buffer.String()).To(ContainSubstring(windowsWarning))
@@ -521,10 +500,7 @@ var _ = Describe("Supply", func() {
 					if len(cmd.Args) > 2 && cmd.Args[1] == "install" {
 						Expect(os.MkdirAll(filepath.Join(cmd.Dir, ".bundle"), 0755)).To(Succeed())
 						Expect(os.WriteFile(filepath.Join(cmd.Dir, ".bundle", "config"), []byte("new bundle config"), 0644)).To(Succeed())
-					} else {
-						return handleBundleBinstubRegeneration(cmd)
 					}
-
 					return nil
 				})
 				Expect(os.WriteFile(filepath.Join(buildDir, "Gemfile"), []byte("source \"https://rubygems.org\"\ngem \"rack\"\n"), 0644)).To(Succeed())
@@ -534,8 +510,6 @@ var _ = Describe("Supply", func() {
 				os.Unsetenv("BUNDLE_CONFIG")
 			})
 
-			itRegeneratesBundleBinstub()
-
 			It("Does not warn the user", func() {
 				Expect(supplier.InstallGems()).To(Succeed())
 				Expect(buffer.String()).ToNot(ContainSubstring(windowsWarning))
@@ -572,8 +546,6 @@ var _ = Describe("Supply", func() {
 						if cmd.Args[1] == "install" {
 							Expect(filepath.Join(cmd.Dir, "Gemfile")).To(BeAnExistingFile())
 							Expect(filepath.Join(cmd.Dir, "Gemfile.lock")).To(BeAnExistingFile())
-						} else {
-							handleBundleBinstubRegeneration(cmd)
 						}
 					})
 					Expect(supplier.InstallGems()).To(Succeed())
@@ -588,8 +560,6 @@ var _ = Describe("Supply", func() {
 						if cmd.Args[1] == "install" {
 							Expect(cmd.Dir).ToNot(Equal(buildDir))
 							installCalled = true
-						} else {
-							handleBundleBinstubRegeneration(cmd)
 						}
 					})
 					Expect(supplier.InstallGems()).To(Succeed())
@@ -614,8 +584,6 @@ var _ = Describe("Supply", func() {
 							Expect(filepath.Join(cmd.Dir, "Gemfile")).To(BeAnExistingFile())
 							Expect(filepath.Join(cmd.Dir, "Gemfile.lock")).ToNot(BeAnExistingFile())
 							Expect(os.WriteFile(filepath.Join(cmd.Dir, "Gemfile.lock"), []byte(newGemfileLock), 0644)).To(Succeed())
-						} else {
-							handleBundleBinstubRegeneration(cmd)
 						}
 					})
 					Expect(supplier.InstallGems()).To(Succeed())
@@ -630,8 +598,6 @@ var _ = Describe("Supply", func() {
 						if cmd.Args[1] == "install" {
 							Expect(cmd.Dir).ToNot(Equal(buildDir))
 							installCalled = true
-						} else {
-							handleBundleBinstubRegeneration(cmd)
 						}
 					})
 					Expect(supplier.InstallGems()).To(Succeed())