Tighten merchant schedule validation

Co-authored-by: StuartFerguson <16325469+StuartFerguson@users.noreply.github.com>

Author: Copilot

TransactionProcessor.Aggregates.Tests/MerchantScheduleAggregateTests.cs

@@ -52,6 +52,18 @@ public void MerchantScheduleAggregate_Create_WithInitialMonths_IsCreated()
             model.Months.Single().OpenDays.ShouldBe([4]);
         }
 
+        [Fact]
+        public void MerchantScheduleAggregate_Create_InvalidYear_ErrorReturned()
+        {
+            MerchantScheduleAggregate aggregate = MerchantScheduleAggregate.Create(MerchantScheduleId);
+
+            Result result = aggregate.Create(EstateId, MerchantId, 1899, []);
+
+            result.IsFailed.ShouldBeTrue();
+            result.Status.ShouldBe(ResultStatus.Invalid);
+            result.Message.ShouldBe("A valid year must be provided when creating a merchant schedule");
+        }
+
         [Fact]
         public void MerchantScheduleAggregate_SetMonthSchedule_WhenChanged_EmitsMonthEvent()
         {

TransactionProcessor.Aggregates/MerchantScheduleAggregate.cs

@@ -149,7 +149,7 @@ private static Result ValidateCreateArguments(MerchantScheduleAggregate aggregat
             if (merchantId == Guid.Empty)
                 return Result.Invalid("Merchant id must be provided when creating a merchant schedule");
 
-            if (year < 1)
+            if (year < 1900)
                 return Result.Invalid("A valid year must be provided when creating a merchant schedule");
 
             return Result.Success();
@@ -194,7 +194,7 @@ public MerchantScheduleAggregate()
         private MerchantScheduleAggregate(Guid aggregateId)
         {
             if (aggregateId == Guid.Empty)
-                throw new ArgumentNullException(nameof(aggregateId));
+                throw new ArgumentException("Value cannot be empty.", nameof(aggregateId));
 
             this.AggregateId = aggregateId;
             this.Months = new Dictionary<Int32, MerchantScheduleMonthModel>();