Document extension fallback maintenance strategy

Added comment explaining that committed extension files serve as
fallback and don't need frequent updates since tests fetch latest
versions. Recommends updating committed files when extension-related
issues occur to document working versions.

(cherry picked from commit 2e6e1fa)

Author: cderv

tests/smoke/render/render-format-extension.test.ts

@@ -19,6 +19,12 @@ import { testRender } from "./render.ts";
 
 // Update extensions to latest versions before testing
 // Falls back to committed versions on network failure
+//
+// Maintenance note: Committed extension files serve as fallback for offline/CI
+// failure scenarios. They don't need frequent updates since tests always fetch
+// latest versions. When extension-related issues occur, consider updating the
+// committed files to document the working version (see commits c3149a9ba and
+// b8421caf9 for example).
 const updateExtensions = async () => {
   try {
     console.log("Updating quarto-journals extensions to latest versions...");