TestDataBuilders: ValidOrder()/ValidClient() with explicit Build(); After tests migrated

Domain objects in After-side tests are constructed only through the
builders, so the upcoming OrderLines evolution changes builder internals
instead of every test. Before keeps object initializers (legacy world).

yak: testdatabuilders-validordervalidclient-after-test-migration-91vl

EXERCISE.md

@@ -22,6 +22,11 @@ explicit — and that is where this exercise lives.
   percentage *actually applied* (0 when the total does not strictly exceed the
   barrier), and the discounted total. Rounding is half-up, in integer math.
 - Unit tests for all of the above already pass.
+- `ValidOrder()` / `ValidClient()` test data builders in the test project
+  (`tests/BeforeAfter.Tests/TestData/`) — construct domain objects in tests
+  through them, so tests survive domain evolution. Import their static
+  factories with `using static BeforeAfter.Tests.TestData.OrderBuilder;`
+  (and `...ClientBuilder;`).
 
 ## The challenge
 
@@ -61,6 +66,8 @@ instead of assuming there is only one order around.
 
 ```csharp
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -74,9 +81,8 @@ public class DiscountedOrderMappingTests
     public void A_discounted_order_can_be_correlated_onto_its_order()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
@@ -98,9 +104,8 @@ public class DiscountedOrderMappingTests
     public void Saving_a_discounted_order_keeps_the_stored_total_untouched()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);