feat: Add route to list accounts that belongs to a platform_app (#12140)

This PR creates a new route to list all accounts that a platform_app has access to.

Fixes: #12109

spec/controllers/platform/api/v1/accounts_controller_spec.rb

@@ -78,6 +78,42 @@ RSpec.describe 'Platform Accounts API', type: :request do
     end
   end
 
+  describe 'GET /platform/api/v1/accounts' do
+    context 'when it is an unauthenticated platform app' do
+      it 'returns unauthorized' do
+        get '/platform/api/v1/accounts'
+        expect(response).to have_http_status(:unauthorized)
+      end
+    end
+
+    context 'when it is an invalid platform app token' do
+      it 'returns unauthorized' do
+        get '/platform/api/v1/accounts', headers: { api_access_token: 'invalid' }, as: :json
+        expect(response).to have_http_status(:unauthorized)
+      end
+    end
+
+    context 'when it is an authenticated platform app' do
+      let(:platform_app) { create(:platform_app) }
+      let!(:account1) { create(:account, name: 'Account A') }
+      let!(:account2) { create(:account, name: 'Account B') }
+
+      before do
+        create(:platform_app_permissible, platform_app: platform_app, permissible: account1)
+        create(:platform_app_permissible, platform_app: platform_app, permissible: account2)
+      end
+
+      it 'returns all permissible accounts' do
+        get '/platform/api/v1/accounts', headers: { api_access_token: platform_app.access_token.token }, as: :json
+
+        expect(response).to have_http_status(:success)
+        json_response = response.parsed_body
+        expect(json_response.size).to eq(2)
+        expect(json_response.map { |acc| acc['name'] }).to include('Account A', 'Account B')
+      end
+    end
+  end
+
   describe 'GET /platform/api/v1/accounts/{account_id}' do
     context 'when it is an unauthenticated platform app' do
       it 'returns unauthorized' do