Update changelogs from fern repo (#487)

Co-authored-by: fearthecowboy <[email protected]>

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2025-08-18.mdx

@@ -0,0 +1,10 @@
+## 2.1.12
+**`(fix):`** Add support for configuration 'experimental-fully-qualified-namespaces' to 
+generate explicit types everywhere. 
+Add support for `experimental-dotnet-format` to have `dotnet format` reduce
+namespaces and namespace prefixes. (this can take a large amount of memory)
+This creates code that is much cleaner, and can easily support
+multiple endpoints with identical names in the generated code without
+name or namespace collisions.
+
+