fix: handle JSON format in render_not_found

Reverts the render_not_found method to use respond_to, matching the pattern
used in the exception handler. This fixes MissingTemplate errors when
requests come in with JSON format (e.g., from API clients).

The previous change (3eeca62) simplified render_not_found but broke JSON
responses. Now HTML requests get the proper 404 page while API requests
get an empty 404 response.

Adds tests for both HTML and JSON format handling.

Author: mroderick

app/controllers/application_controller.rb

@@ -30,7 +30,10 @@ class ApplicationController < ActionController::Base
   before_action :accept_terms, if: :logged_in?
 
   def render_not_found
-    render template: 'errors/not_found', layout: false, status: :not_found
+    respond_to do |format|
+      format.html { render template: 'errors/not_found', layout: false, status: :not_found }
+      format.all { head :not_found }
+    end
   end
 
   protected

spec/controllers/application_controller_spec.rb

@@ -0,0 +1,31 @@
+RSpec.describe ApplicationController do
+  describe '#render_not_found' do
+    controller do
+      def index
+        raise ActiveRecord::RecordNotFound
+      end
+    end
+
+    context 'with HTML format' do
+      before do
+        get :index, format: :html
+      end
+
+      it 'renders the not_found template' do
+        expect(response.status).to eq(404)
+        expect(response).not_to be_redirect
+      end
+    end
+
+    context 'with JSON format' do
+      before do
+        get :index, format: :json
+      end
+
+      it 'returns empty 404 response' do
+        expect(response.status).to eq(404)
+        expect(response.body).to be_empty
+      end
+    end
+  end
+end