fix: only allow afterId or beforeId to be set. not both at same time.

tests/AnthropicClient.Tests/Integration/AnthropicApiClientTests.cs

@@ -549,13 +549,12 @@ public class AnthropicApiClientTests : IntegrationTest
   [Fact]
   public async Task ListModelsAsync_WhenCalledWithPagingRequestUsingCustomValues_ItShouldReturnListOfModels()
   {
-    var pagingRequest = new PagingRequest("prev_id", "next_id", 10);
+    var pagingRequest = new PagingRequest(afterId: "next_id", limit: 10);
 
     _mockHttpMessageHandler
       .WhenListModelsRequest()
       .WithQueryString(new Dictionary<string, string>
       {
-        { "before_id", pagingRequest.BeforeId },
         { "after_id", pagingRequest.AfterId },
         { "limit", pagingRequest.Limit.ToString() },
       })

tests/AnthropicClient.Tests/Unit/Models/PagingRequestTests.cs

@@ -18,6 +18,14 @@ public class PagingRequestTests : SerializationTest
     act.Should().Throw<ArgumentOutOfRangeException>().WithMessage("limit must be between 1 and 1000. (Parameter 'limit')");
   }
 
+  [Fact]
+  public void Constructor_WhenBothBeforeIdAndAfterIdAreSet_ItShouldThrowArgumentException()
+  {
+    var act = () => new PagingRequest(beforeId: "before-id", afterId: "after-id");
+
+    act.Should().Throw<ArgumentException>().WithMessage("Only one of beforeId or afterId can be set.");
+  }
+
   [Fact]
   public void ToQueryParameters_WhenNoPropertiesSet_ItShouldReturnEmptyString()
   {
@@ -57,14 +65,4 @@ public class PagingRequestTests : SerializationTest
 
     result.Should().Be("limit=10");
   }
-
-  [Fact]
-  public void ToQueryParameters_WhenAllPropertiesAreSet_ItShouldReturnAllProperties()
-  {
-    var pagingRequest = new PagingRequest(beforeId: "before-id", afterId: "after-id", limit: 10);
-
-    var result = pagingRequest.ToQueryParameters();
-
-    result.Should().Be("before_id=before-id&after_id=after-id&limit=10");
-  }
 }