Update changelogs from fern repo (#4745)

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

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-04-03.mdx

@@ -0,0 +1,14 @@
+## 2.56.5
+**`(fix):`** Fix mock server test generation to omit null values for non-nullable
+properties in inlined request bodies, matching the SDK's
+JsonIgnoreCondition.WhenWritingNull serialization behavior. When
+enableExplicitNullableOptional is disabled (default), all nulls are
+now unconditionally omitted from expected request JSON.
+
+
+**`(fix):`** Fix pagination test build errors (CS8411) when generatePaginatedClients
+is disabled. The test generator now checks the config flag before emitting
+`await foreach` code, preventing attempts to iterate over plain response
+types that do not implement IAsyncEnumerable.
+
+