Update changelogs from fern repo (#4261)

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-03-13.mdx

@@ -1,3 +1,13 @@
+## 2.29.0
+**`(feat):`** Rework enum JSON serialization to eliminate reflection. Each enum now gets
+a generated serializer with static dictionary lookups instead of the generic
+`EnumSerializer<T>` / `StringEnumSerializer<T>` that used `Enum.GetValues`,
+`GetField`, `GetCustomAttributes`, and `Activator.CreateInstance` at runtime.
+Serializer initialization is ~1,000x faster (18 ns vs 18 us) and allocates
+160x less memory; steady-state throughput is unchanged. The generated code is
+also NativeAOT and IL-trimming compatible.
+
+
 ## 2.28.0
 **`(feat):`** Generate a separate WireMock server for each test fixture instead of sharing
 a single global server across all mock server tests. Each test class now starts