Update changelogs from fern repo (#4581)

Co-authored-by: iamnamananand996 <31537362+iamnamananand996@users.noreply.github.com>

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-03-26.mdx

@@ -0,0 +1,8 @@
+## 2.54.7
+**`(fix):`** Wire Basic Auth credentials into the Authorization header. Previously,
+the generated client accepted username/password constructor parameters
+but never set the `Authorization: Basic <base64>` header on outgoing
+requests. The constructor now encodes the credentials and adds the
+header automatically.
+
+