Update changelogs from fern repo (#3323)

Co-authored-by: Swimburger <3382717+Swimburger@users.noreply.github.com>

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-01-27.mdx

@@ -1,3 +1,40 @@
+## 2.19.0
+**`(feat):`** Add support for type-safe undiscriminated unions with the new `use-undiscriminated-unions` configuration option.
+
+Undiscriminated unions provide a clean, type-safe way to work with values that can be one of several types:
+
+```csharp
+// Creating union values
+var response = MyUnion.FromString("hello");
+var response = MyUnion.FromInt(42);
+var response = MyUnion.FromCustomObject(new CustomObject { ... });
+
+// Type-safe value access with Is* properties and As* methods
+if (response.IsString)
+{
+    string value = response.AsString();
+    Console.WriteLine($"Got string: {value}");
+}
+else if (response.IsInt)
+{
+    int value = response.AsInt();
+    Console.WriteLine($"Got integer: {value}");
+}
+
+// Safe extraction with TryAs* pattern (no exceptions)
+if (response.TryAsCustomObject(out var obj))
+{
+    Console.WriteLine($"Got object: {obj.Name}");
+}
+```
+
+This provides better IntelliSense support and compile-time safety compared to working with generic `object` types.
+
+
+**`(chore):`** Reduce NuGet package dependencies. The OneOf library is no longer required when undiscriminated unions are not used,
+resulting in smaller package sizes and fewer transitive dependencies.
+
+
 ## 2.18.1
 **`(fix):`** While most `<`s and `>`s should be escaped to `&lt;` and `&gt;` to avoid mangling the output,
 genuine XML/HTML tags should be kept as-is; this commit scans for potential valid tags and