ci: dedupe validate into package, add caches, run phpunit in the shopware-cli image

- drop the separate validate job (the zip is validated after packaging); same in release
- shared scripts/ci/assert-built-assets.sh (BusyBox-safe), now also used by release
- actions/cache for npm, shopware-cli and composer
- phpunit: shopware-cli image instead of a 4.5 min apt install; MariaDB 10.11
  (11.4 broke Shopware's migrations: Duplicate column 'sorter_key')
- protect-main.sh: drop the removed validate context

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/protect-main.sh

@@ -10,7 +10,6 @@ tea api -X PATCH "repos/${REPO}/branch_protections/main" -d '{
   "enable_status_check": true,
   "status_check_contexts": [
     "version-changelog / version-changelog (pull_request)",
-    "plugin-checks / validate (pull_request)",
     "plugin-checks / package (pull_request)",
     "plugin-checks / phpunit (pull_request)"
   ]