Preserve wizard completion notices (#967)

## Summary
- ensure wizard completion redirects include success notice and keep it
for turbo redirects
- add request spec verifying notice persists on private platforms

## Testing
- `bin/ci` *(fails: bundler: command not found: rails)*
- `bundle exec rubocop` *(fails: bundler: command not found: rubocop)*
- `bundle exec brakeman -q -w2` *(fails: bundler: command not found:
brakeman)*
- `bundle exec bundler-audit --update` *(fails: bundler: command not
found: bundler-audit)*
- `bin/codex_style_guard` *(fails: bundler: command not found: rubocop)*

------
https://chatgpt.com/codex/tasks/task_e_689a50d0093c8321834a9a87c24af10c

Author: rsmithlal

app/controllers/concerns/better_together/wizard_methods.rb

@@ -15,9 +15,8 @@ def determine_wizard_outcome # rubocop:todo Metrics/AbcSize
       raise StandardError, "Wizard #{wizard_identifier} was not found. Have you run the seeds?" unless wizard
 
       if wizard.completed?
-        flash[:notice] = wizard.success_message
-        # TODO: This needs to be adjusted for private platforms. Flash message is not retained after wizard completion
-        redirect_to wizard.success_path
+        flash.keep(:notice)
+        redirect_to wizard.success_path, notice: wizard.success_message
       else
         next_step_path, flash_key, message = wizard_next_step_info
         flash[flash_key] = message if message

spec/requests/better_together/wizard_completion_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Setup Wizard completion' do
+  let!(:wizard) { BetterTogether::Wizard.find_by!(identifier: 'host_setup') }
+
+  before do
+    wizard.mark_completed
+  end
+
+  it 'redirects to the success path with notice preserved' do # rubocop:disable RSpec/MultipleExpectations
+    get better_together.setup_wizard_path(locale: I18n.locale)
+    expect(response).to redirect_to(wizard.success_path)
+
+    # The controller preserves the notice in the flash; assert it directly
+    expect(flash[:notice]).to eq(wizard.success_message)
+  end
+end