Add == and != test cases for Instant and Instant? comparison tests

Agent-Logs-Url: https://github.com/zzzprojects/System.Linq.Dynamic.Core/sessions/4e543367-6cb4-4164-bd33-fbf0fecca256

Co-authored-by: StefH <249938+StefH@users.noreply.github.com>

Author: Copilot

test/System.Linq.Dynamic.Core.Tests/TypeConvertors/NodaTimeConverterTests.cs

@@ -169,6 +169,8 @@ private class EntityWithInstant
         [InlineData(">=", 2)]
         [InlineData("<", 1)]
         [InlineData("<=", 2)]
+        [InlineData("==", 1)]
+        [InlineData("!=", 2)]
         public void FilterByInstant_WithRelationalOperator(string op, int expectedCount)
         {
             // Arrange
@@ -192,6 +194,8 @@ public void FilterByInstant_WithRelationalOperator(string op, int expectedCount)
         [InlineData(">=", 2)]
         [InlineData("<", 1)]
         [InlineData("<=", 2)]
+        [InlineData("==", 1)]
+        [InlineData("!=", 3)] // null != now evaluates to true in C# nullable semantics
         public void FilterByNullableInstant_WithRelationalOperator(string op, int expectedCount)
         {
             // Arrange