Update changelogs from fern repo (#4414)

Author: fern-support

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

@@ -1,3 +1,10 @@
+## 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
+`UnsubscribeAll` are synchronized to prevent `InvalidOperationException` when
+handlers are added or removed concurrently from different threads.
+
+
 ## 2.38.0
 **`(feat):`** Add optional `CancellationToken` parameter to all public async WebSocket methods
 (`ConnectAsync`, `CloseAsync`, and `Send` overloads). The token is threaded through