Fix brew bundle dump --global to respect XDG_CONFIG_HOME

- Remove File.exist? check in brewfile.rb to use XDG path even when Brewfile doesn't exist yet
- Add directory creation in dumper.rb to ensure parent directories exist before writing
- Add test case for XDG behavior when Brewfile doesn't exist initially

Co-authored-by: MikeMcQuaid <125011+MikeMcQuaid@users.noreply.github.com>

Library/Homebrew/bundle/brewfile.rb

@@ -24,7 +24,7 @@ module Homebrew
           else
             raise "'HOMEBREW_BUNDLE_FILE' cannot be specified with '--global'" if env_bundle_file.present?
 
-            if user_config_home && File.exist?("#{user_config_home}/Brewfile")
+            if user_config_home
               "#{user_config_home}/Brewfile"
             else
               Bundle.exchange_uid_if_needed! do

Library/Homebrew/bundle/dumper.rb

@@ -81,6 +81,7 @@ module Homebrew
       sig { params(file: Pathname, content: String).void }
       def self.write_file(file, content)
         Bundle.exchange_uid_if_needed! do
+          FileUtils.mkdir_p file.parent
           file.open("w") { |io| io.write content }
         end
       end

Library/Homebrew/test/bundle/brewfile_spec.rb

@@ -1,3 +1,4 @@
+# typed: strict
 # frozen_string_literal: true
 
 require "bundle"
@@ -171,6 +172,14 @@ RSpec.describe Homebrew::Bundle::Brewfile do
           expect(path).to eq(expected_pathname)
         end
       end
+
+      context "when HOMEBREW_USER_CONFIG_HOME is set but Brewfile does not exist" do
+        let(:config_dir_brewfile_exist) { false }
+
+        it "returns the XDG-compliant path when HOMEBREW_USER_CONFIG_HOME is set" do
+          expect(path).to eq(Pathname.new("#{env_user_config_home_value}/Brewfile"))
+        end
+      end
     end
 
     context "when HOMEBREW_BUNDLE_FILE has a value" do