ci: fix hanging setup-php, wait for DB manually, add compatibility_date

- validate job needs no PHP (the package job already validates without it)
- phpunit: runner=self-hosted so setup-php does not hang under act; replace the
  unhealthy service healthcheck with a TCP wait loop
- .config/shopware-extension.yml: set compatibility_date

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

.config/shopware-extension.yml

@@ -1,3 +1,4 @@
+compatibility_date: 2026-02-11
 # shopware-cli extension configuration (build/package/validate)
 build:
   zip: