fix: throw exception when accessing result props incorrectly

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

@@ -11,7 +11,12 @@ public class AnthropicResultTests
     var actual = AnthropicResult<string>.Success(value, headers);
 
     actual.IsSuccess.Should().BeTrue();
+    actual.IsFailure.Should().BeFalse();
     actual.Value.Should().Be(value);
+
+    var action = () => { var _ = actual.Error; };
+    action.Should().Throw<InvalidOperationException>();
+
     actual.Headers.Should().Be(headers);
   }
 
@@ -24,7 +29,12 @@ public class AnthropicResultTests
     var actual = AnthropicResult<string>.Failure(error, headers);
 
     actual.IsSuccess.Should().BeFalse();
+    actual.IsFailure.Should().BeTrue();
     actual.Error.Should().Be(error);
+    
+    var action = () => { var _ = actual.Value; };
+    action.Should().Throw<InvalidOperationException>();
+
     actual.Headers.Should().Be(headers);
   }
 }