test: achieve 100% line and branch coverage

Add missing tests for validation logic branches:

- CreditCard: invalid Luhn checksum test (valid length but wrong checksum)

- CustomRule: default error message when null (line 63 null coalescing)

- RuleBuilder: null propertyValidator defensive test

- ValidationResult.AddError: null error defensive test

- Unless: defensive test for missing rule

Coverage: 280/280 lines (100%), 184/184 branches (100%)

Author: Akácz Károly

tests/Fox.ValidationKit.Tests/CommonValidatorTests.cs

@@ -165,6 +165,18 @@ public void CreditCard_should_fail_for_null()
         result.Errors.Should().Contain(e => e.ErrorCode == ValidationErrorCodes.NotNull);
     }
 
+    [Fact]
+    public void CreditCard_should_fail_for_invalid_luhn_checksum()
+    {
+        var validator = new ContactInfoValidator();
+        var contact = new ContactInfo { Email = "test@example.com", Website = "https://example.com", CreditCard = "4532015112830367" };
+
+        var result = validator.Validate(contact);
+
+        result.IsValid.Should().BeFalse();
+        result.Errors.Should().Contain(e => e.PropertyName == "CreditCard" && e.ErrorCode == ValidationErrorCodes.CreditCard);
+    }
+
     //==============================================================================================
     /// <summary>
     /// Test model for Length validation.

tests/Fox.ValidationKit.Tests/ConditionalValidationTests.cs

@@ -196,6 +196,11 @@ public void CallWhenWithoutRule()
         {
             RuleFor(x => x.CompanyName).When(x => x.IsCompany);
         }
+
+        public void CallUnlessWithoutRule()
+        {
+            RuleFor(x => x.CompanyName).Unless(x => x.IsCompany);
+        }
     }
 
     //==============================================================================================
@@ -212,4 +217,19 @@ public void When_should_throw_when_called_without_rule()
 
         act.Should().Throw<InvalidOperationException>().WithMessage("*must be called after*");
     }
+
+    //==============================================================================================
+    /// <summary>
+    /// Tests that Unless throws when called without a preceding rule.
+    /// </summary>
+    //==============================================================================================
+    [Fact]
+    public void Unless_should_throw_when_called_without_rule()
+    {
+        var validator = new EmptyValidator();
+
+        var act = () => validator.CallUnlessWithoutRule();
+
+        act.Should().Throw<InvalidOperationException>().WithMessage("*must be called after*");
+    }
 }

tests/Fox.ValidationKit.Tests/InternalRulesDefensiveCodeTests.cs

@@ -99,6 +99,23 @@ public void CustomRule_should_throw_for_null_asyncPredicate()
             .Which.ParamName.Should().Be("asyncPredicate");
     }
 
+    //==============================================================================================
+    /// <summary>
+    /// Tests that CustomRule uses default error message when errorMessage is null.
+    /// </summary>
+    //==============================================================================================
+    [Fact]
+    public void CustomRule_should_use_default_error_message_when_null()
+    {
+        var rule = new CustomRule<TestModel, string?>("Value", (instance, value) => false, null!);
+        var model = new TestModel { Value = "test" };
+
+        var errors = rule.Validate(model, "test").ToList();
+
+        errors.Should().ContainSingle();
+        errors[0].Message.Should().Be("Value is invalid.");
+    }
+
     //==============================================================================================
     /// <summary>
     /// Tests that RuleForEachRule constructor throws for null propertyName.

tests/Fox.ValidationKit.Tests/PropertyValidatorTests.cs

@@ -66,3 +66,34 @@ public void AddRule_should_throw_for_null_rule()
         act.Should().Throw<ArgumentNullException>().WithParameterName("rule");
     }
 }
+
+//==================================================================================================
+/// <summary>
+/// Tests for RuleBuilder defensive null checks.
+/// </summary>
+//==================================================================================================
+public sealed class RuleBuilderTests
+{
+    //==============================================================================================
+    /// <summary>
+    /// Test model for RuleBuilder tests.
+    /// </summary>
+    //==============================================================================================
+    private sealed class TestModel
+    {
+        public int Value { get; set; }
+    }
+
+    //==============================================================================================
+    /// <summary>
+    /// Tests that RuleBuilder constructor throws for null propertyValidator.
+    /// </summary>
+    //==============================================================================================
+    [Fact]
+    public void Constructor_should_throw_for_null_propertyValidator()
+    {
+        var act = () => new RuleBuilder<TestModel, int>(null!);
+
+        act.Should().Throw<ArgumentNullException>().WithParameterName("propertyValidator");
+    }
+}

tests/Fox.ValidationKit.Tests/ValidationResultDefensiveCodeTests.cs

@@ -34,4 +34,22 @@ public void Constructor_should_throw_for_null_errors()
             .WithInnerException<ArgumentNullException>()
             .Which.ParamName.Should().Be("errors");
     }
+
+    //==============================================================================================
+    /// <summary>
+    /// Tests that ValidationResult.AddError throws for null error.
+    /// </summary>
+    //==============================================================================================
+    [Fact]
+    public void AddError_should_throw_for_null_error()
+    {
+        var result = ValidationResult.Success();
+        var addErrorMethod = typeof(ValidationResult).GetMethod("AddError", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
+
+        var act = () => addErrorMethod!.Invoke(result, [null!]);
+
+        act.Should().Throw<System.Reflection.TargetInvocationException>()
+            .WithInnerException<ArgumentNullException>()
+            .Which.ParamName.Should().Be("error");
+    }
 }