📝 **Task:** Implement `validate_migration_plan(plan)` returning a sorted list of issues from `{'combined_expand_and_contract','backfill_before_expand','drop_before_read_switch'}`. `plan` is a list of deploys; each deploy is a list of operation strings. 📋 Implement the function above. Tests run automatically. 💡 **Hint:** Re-read the theory if you get stuck.