Add unit tests for EnvelopeSigningType.Normalize method

Added ConstantsTests to verify Normalize behavior for EnvelopeSigningType, including handling of valid and out-of-range enum values using NUnit test cases. Ensures normalization logic works as expected.

EnvelopeGenerator.Tests/Domain/ConstantsTests.cs

@@ -0,0 +1,17 @@
+using EnvelopeGenerator.Domain.Constants;
+using NUnit.Framework;
+
+namespace EnvelopeGenerator.Tests.Domain;
+
+public class ConstantsTests
+{
+    [TestCase(EnvelopeSigningType.ReadAndSign, EnvelopeSigningType.ReadAndSign)]
+    [TestCase(EnvelopeSigningType.WetSignature, EnvelopeSigningType.WetSignature)]
+    [TestCase((EnvelopeSigningType)5, EnvelopeSigningType.WetSignature)]
+    public void Normalize_ReturnsExpectedValue(EnvelopeSigningType input, EnvelopeSigningType expected)
+    {
+        var normalized = input.Normalize();
+
+        Assert.That(normalized, Is.EqualTo(expected));
+    }
+}