Add vitest dependency grouping to dependabot config

Groups vitest and @vitest/* packages together for cleaner PRs

Author: eckardt

.github/dependabot.yml

@@ -12,6 +12,11 @@ updates:
     commit-message:
       prefix: "security"
       include: "scope"
+    groups:
+      vitest:
+        patterns:
+          - "vitest*"
+          - "@vitest/*"
     ignore:
       # Keep @types/node aligned with minimum supported Node.js version (20)
       - dependency-name: "@types/node"