Add changelog entry as regression fix

[skip ci]

Author: cderv

news/changelog-1.9.md

@@ -6,6 +6,7 @@ All changes included in 1.9:
 - ([#13441](https://github.com/quarto-dev/quarto-cli/pull/13441)): Catch `undefined` exceptions in Pandoc failure to avoid spurious error message.
 - ([#13046](https://github.com/quarto-dev/quarto-cli/issues/13046)): Use new url for multiplex socket.io server <https://multiplex.up.railway.app/> as default for `format: revealjs` and `revealjs.multiplex: true`.
 - ([#13506](https://github.com/quarto-dev/quarto-cli/issues/13506)): Fix navbar active state detection when sidebar has no logo configured. Prevents empty logo links from interfering with navigation highlighting.
+- ([#13616](https://github.com/quarto-dev/quarto-cli/issues/13616)): Fix fatal error when rendering manuscript projects with custom blocks like ConditionalBlock.
 - ([#13625](https://github.com/quarto-dev/quarto-cli/issues/13625)): Fix Windows file locking error (os error 32) when rendering with `--output-dir` flag. Context cleanup now happens before removing the temporary `.quarto` directory, ensuring file handles are properly closed.
 - ([#13633](https://github.com/quarto-dev/quarto-cli/issues/13633)): Fix detection and auto-installation of babel language packages from newer error format that doesn't explicitly mention `.ldf` filename.