Fix phpstan errors in "check (highest)" job (deprecated EntitySearchResult accessors, MJML offset)

[Copilot]

The check (highest) job runs shopware-cli extension validate against the highest Shopware version and fails with 5 phpstan errors: one possibly-missing array offset and four calls to accessors deprecated in 6.8.

• MjmlLoader

  • $compileTemplate['html'] may be unset according to phpstan's inferred type (array{errors?: array{}, html?: string}).
  • It now returns $compileTemplate['html'] ?? ''.
  • '' is the existing "fall back to Shopware default templates" value used earlier in the same method.

• SearchPathProvider and FlowSubscriber

  • EntitySearchResult::getElements() and first() are deprecated since 6.8.
  • Calls now go through getEntities():

// before
$repo->search($criteria, $context)->first();
// after
$repo->search($criteria, $context)->getEntities()->first();

Review note: composer.json still allows ~6.5.8 || ~6.6.0 || ~6.7.0. I haven't confirmed that EntitySearchResult::getEntities() exists in all of those versions. If it doesn't, either narrow the constraint or add a compatibility shim.

The services.xml deprecation warning from the same run is not addressed here.