Update changelogs from fern repo (#4417)

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

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-03-20.mdx

@@ -1,3 +1,11 @@
+## 2.40.0
+**`(feat):`** Configure PING/PONG keep-alive on `ClientWebSocket`. The default connection factory
+now sets `KeepAliveInterval` (default: 30 s) and, on .NET 9+, `KeepAliveTimeout`
+(default: 20 s) to enable the PING/PONG keep-alive strategy that detects unresponsive
+servers at the transport level. Both values are exposed as public properties on
+`WebSocketConnection` for caller customization.
+
+
 ## 2.38.2
 **`(fix):`** Add `using` declaration to `JsonDocument` in WebSocket `OnTextMessage` handler.
 `JsonDocument` implements `IDisposable` and holds pooled memory buffers; without