ci: update presubmit validation (gemini-cli-extensions#100)

Author: averikitsch

.github/workflows/presubmit-tests.yml

@@ -34,6 +34,6 @@ jobs:
           chmod +x toolbox
 
       - name: Install Extension
+        run: npx gemini extensions validate .
         env:
           GEMINI_API_KEY: "placeholder"
-        run: yes | npx gemini extensions install .