Update changelogs from fern repo (#4463)

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

Author: fern-support

fern/products/cli-api-reference/cli-changelog/2026-03-20.mdx

@@ -1,3 +1,14 @@
+## 4.39.0
+**`(feat):`** Add `--require-env-vars` flag to `fern generate`. Use `--no-require-env-vars`
+to substitute empty strings for undefined environment variables instead of
+causing an error. Defined environment variables are still resolved normally.
+This is useful for local development when registry credentials are not needed.
+
+```bash
+fern generate --group csharp-sdk --no-require-env-vars
+```
+
+
 ## 4.38.0
 **`(feat):`** Auto-detect CI environment (GitHub Actions, GitLab CI, Bitbucket Pipelines)
 during `fern generate --docs` and send source metadata (repository, run ID,