fix: throw exception when accessing result props incorrectly

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

@@ -0,0 +1,34 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class ToolCallResultTests
+{
+  [Fact]
+  public void Success_WhenCalled_ItShouldReturnSuccessResult()
+  {
+    var value = "success";
+
+    var actual = ToolCallResult<string>.Success(value);
+
+    actual.IsSuccess.Should().BeTrue();
+    actual.IsFailure.Should().BeFalse();
+    actual.Value.Should().Be(value);
+
+    var action = () => { var _ = actual.Error; };
+    action.Should().Throw<InvalidOperationException>();
+  }
+
+  [Fact]
+  public void Failure_WhenCalled_ItShouldReturnFailureResult()
+  {
+    var error = new Exception();
+
+    var actual = ToolCallResult<string>.Failure(error);
+
+    actual.IsSuccess.Should().BeFalse();
+    actual.IsFailure.Should().BeTrue();
+    actual.Error.Should().Be(error);
+    
+    var action = () => { var _ = actual.Value; };
+    action.Should().Throw<InvalidOperationException>();
+  }
+}