Add JSON format support to signup flow for native API clients

The magic link verification endpoint now returns `requires_signup_completion`
to indicate whether the client needs to collect a name and complete signup.
The signup completion endpoint now supports JSON responses with the account ID.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/controllers/sessions/magic_links_controller_test.rb

@@ -80,7 +80,7 @@ class Sessions::MagicLinksControllerTest < ActionDispatch::IntegrationTest
     assert MagicLink.exists?(expired_link.id), "Expired magic link should not be consumed"
   end
 
-  test "create via JSON" do
+  test "create via JSON for sign in" do
     identity = identities(:david)
     magic_link = identity.send_magic_link
 
@@ -89,6 +89,20 @@ class Sessions::MagicLinksControllerTest < ActionDispatch::IntegrationTest
       post session_magic_link_path(format: :json), params: { code: magic_link.code }
       assert_response :success
       assert @response.parsed_body["session_token"].present?
+      assert_equal false, @response.parsed_body["requires_signup_completion"]
+    end
+  end
+
+  test "create via JSON for sign up" do
+    identity = identities(:david)
+    magic_link = identity.send_magic_link(for: :sign_up)
+
+    untenanted do
+      post session_path(format: :json), params: { email_address: identity.email_address }
+      post session_magic_link_path(format: :json), params: { code: magic_link.code }
+      assert_response :success
+      assert @response.parsed_body["session_token"].present?
+      assert_equal true, @response.parsed_body["requires_signup_completion"]
     end
   end
 

test/controllers/signup/completions_controller_test.rb

@@ -55,4 +55,30 @@ class Signup::CompletionsControllerTest < ActionDispatch::IntegrationTest
       assert_select "li", text: "Full name can't be blank"
     end
   end
+
+  test "create via JSON" do
+    untenanted do
+      post signup_completion_path(format: :json), params: {
+        signup: {
+          full_name: @signup.full_name
+        }
+      }
+    end
+
+    assert_response :created
+    assert @response.parsed_body["account_id"].present?
+  end
+
+  test "create via JSON with blank name" do
+    untenanted do
+      post signup_completion_path(format: :json), params: {
+        signup: {
+          full_name: ""
+        }
+      }
+    end
+
+    assert_response :unprocessable_entity
+    assert_includes @response.parsed_body["errors"], "Full name can't be blank"
+  end
 end