Fix IPv4 format validator regex false positives (#1909)

Escape dot character in regex to match literal dots only,
preventing strings like '00:45:00.0' from passing validation.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: lahma

src/NJsonSchema.Tests/Validation/FormatIpV4Tests.cs

@@ -38,5 +38,56 @@ public void When_format_ipv4_correct_then_validation_succeeds()
             // Assert
             Assert.Empty(errors);
         }
+
+        [Fact]
+        public void When_format_ipv4_has_colons_instead_of_dots_then_validation_fails()
+        {
+            // Arrange
+            var schema = new JsonSchema();
+            schema.Type = JsonObjectType.String;
+            schema.Format = JsonFormatStrings.IpV4;
+
+            var token = new JValue("00:45:00.0");
+
+            // Act
+            var errors = schema.Validate(token);
+
+            // Assert
+            Assert.Equal(ValidationErrorKind.IpV4Expected, errors.First().Kind);
+        }
+
+        [Fact]
+        public void When_format_ipv4_has_colons_as_separators_then_validation_fails()
+        {
+            // Arrange
+            var schema = new JsonSchema();
+            schema.Type = JsonObjectType.String;
+            schema.Format = JsonFormatStrings.IpV4;
+
+            var token = new JValue("1:2:3:4");
+
+            // Act
+            var errors = schema.Validate(token);
+
+            // Assert
+            Assert.Equal(ValidationErrorKind.IpV4Expected, errors.First().Kind);
+        }
+
+        [Fact]
+        public void When_format_ipv4_octet_exceeds_255_then_validation_fails()
+        {
+            // Arrange
+            var schema = new JsonSchema();
+            schema.Type = JsonObjectType.String;
+            schema.Format = JsonFormatStrings.IpV4;
+
+            var token = new JValue("256.1.1.1");
+
+            // Act
+            var errors = schema.Validate(token);
+
+            // Assert
+            Assert.Equal(ValidationErrorKind.IpV4Expected, errors.First().Kind);
+        }
     }
 }

src/NJsonSchema/Validation/FormatValidators/IpV4FormatValidator.cs

@@ -14,7 +14,7 @@ namespace NJsonSchema.Validation.FormatValidators
     /// <summary>Validator for "IpV4" format.</summary>
     public class IpV4FormatValidator : IFormatValidator
     {
-        private const string IpV4RegexExpression = @"^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$";
+        private const string IpV4RegexExpression = @"^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$";
 
         /// <summary>Gets the format attribute's value.</summary>
         public string Format { get; } = JsonFormatStrings.IpV4;