Fix JsonSourceGenerator verbatim identifier handling in property initializers

Fix #116507: C# verbatim identifiers like @else are now correctly handled
when generating property initializers for init-only properties. The fix
changes PropertyInitializerGenerationSpec.Name to use NameSpecifiedInSourceCode
(which includes the @ prefix for reserved keywords) instead of MemberName.

Also added a unit test to verify this scenario works correctly.

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

@@ -1572,7 +1572,7 @@ private void ProcessMember(
 
                             var propertyInitializer = new PropertyInitializerGenerationSpec
                             {
-                                Name = property.MemberName,
+                                Name = property.NameSpecifiedInSourceCode,
                                 ParameterType = property.PropertyType,
                                 MatchesConstructorParameter = matchingConstructorParameter is not null,
                                 ParameterIndex = matchingConstructorParameter?.ParameterIndex ?? paramCount++,

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

@@ -861,6 +861,33 @@ internal partial class ModelContext : JsonSerializerContext
             CompilationHelper.RunJsonSourceGenerator(compilation, logger: logger);
         }
 
+#if ROSLYN4_4_OR_GREATER && NET
+        [Fact]
+        public void InitOnlyPropertyWithReservedKeywordName_CompilesSuccessfully()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/116507
+            // Verbatim identifiers like @else should be correctly handled in property initializers.
+
+            string source = """
+                #nullable enable
+                using System.Text.Json.Serialization;
+
+                public record A
+                {
+                    public A? @else { get; init; }
+                }
+
+                [JsonSerializable(typeof(A))]
+                public partial class MyContext : JsonSerializerContext
+                {
+                }
+                """;
+
+            Compilation compilation = CompilationHelper.CreateCompilation(source);
+            CompilationHelper.RunJsonSourceGenerator(compilation, logger: logger);
+        }
+#endif
+
         [Fact]
         public void RefStructPropertyWithJsonIgnore_CompilesSuccessfully()
         {