Update changelogs from fern repo (#4416)

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

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-03-19.mdx

@@ -1,3 +1,11 @@
+## 2.39.0
+**`(feat):`** Generate interfaces for WebSocket API classes (e.g., `IStreamApi`, `ITranscribeApi`)
+to improve testability. Each WebSocket client now implements a corresponding interface
+that declares all public event properties, `Status`, `ConnectAsync`, `Send`, and
+`CloseAsync`. The root client interface (`ICortiClient`) also includes factory method
+signatures (`CreateStreamApi`, `CreateTranscribeApi`) when WebSockets are enabled.
+
+
 ## 2.38.1
 **`(fix):`** Fix `Event<T>` thread-safety issue. `RaiseEvent` now snapshots the subscriber
 collections under a lock before iterating, and `Subscribe`, `Unsubscribe`, and