Small native API changes: include identity ID, return empty signup completion body

Include identity.id in the my/identity.json response and return an
empty body from the CREATE signup/completions.json endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/controllers/my/identities_controller_test.rb

@@ -11,6 +11,7 @@ class My::IdentitiesControllerTest < ActionDispatch::IntegrationTest
     untenanted do
       get my_identity_path, as: :json
       assert_response :success
+      assert_equal identity.id, @response.parsed_body["id"]
       assert_equal identity.accounts.count, @response.parsed_body["accounts"].count
     end
   end

test/controllers/signup/completions_controller_test.rb

@@ -68,7 +68,6 @@ class Signup::CompletionsControllerTest < ActionDispatch::IntegrationTest
     end
 
     assert_response :created
-    assert_equal Account.last.id, @response.parsed_body["account_id"]
   end
 
   test "create via JSON with blank name" do