Fix debug platform inclusion for Windows

lib/ruby_lsp/setup_bundler.rb

@@ -152,7 +152,13 @@ def write_custom_gemfile
       end
 
       unless @dependencies["debug"]
-        parts << 'gem "debug", require: false, group: :development, platforms: :mri'
+        # The `mri` platform excludes Windows. We want to install the debug gem only on MRI for any operating system,
+        # but that constraint doesn't yet exist in Bundler. On Windows, we are manually checking if the engine is MRI
+        parts << if Gem.win_platform?
+          'gem "debug", require: false, group: :development, install_if: -> { RUBY_ENGINE == "ruby" }'
+        else
+          'gem "debug", require: false, group: :development, platforms: :mri'
+        end
       end
 
       if @rails_app && !@dependencies["ruby-lsp-rails"]
@@ -238,9 +244,13 @@ def run_bundle_install(bundle_gemfile = @gemfile)
     sig { params(env: T::Hash[String, String]).returns(T::Hash[String, String]) }
     def run_bundle_install_directly(env)
       RubyVM::YJIT.enable if defined?(RubyVM::YJIT.enable)
+
+      # The ENV can only be merged after checking if an update is required because we depend on the original value of
+      # ENV["BUNDLE_GEMFILE"], which gets overridden after the merge
+      should_update = should_bundle_update?
       T.unsafe(ENV).merge!(env)
 
-      unless should_bundle_update?
+      unless should_update
         Bundler::CLI::Install.new({}).run
         correct_relative_remote_paths if @custom_lockfile.exist?
         return env

test/integration_test.rb

@@ -145,6 +145,22 @@ def test_launch_mode_with_no_gemfile_and_bundle_path
     end
   end
 
+  def test_composed_bundle_includes_debug
+    in_temp_dir do |dir|
+      Bundler.with_unbundled_env do
+        launch(dir)
+
+        _stdout, stderr = capture_subprocess_io do
+          system(
+            { "BUNDLE_GEMFILE" => File.join(dir, ".ruby-lsp", "Gemfile") },
+            "bundle exec ruby -e 'require \"debug\"'",
+          )
+        end
+        assert_empty(stderr)
+      end
+    end
+  end
+
   private
 
   def launch(workspace_path)