Fix zone hydration under service areas

[roncodes]

Summary

• Fix service-area/zone Eloquent relationship keys to use zones.service_area_uuid -> service_areas.uuid
• Add regression coverage for the relationship contract and public API service_area key

Root Cause

Zone creation persisted service_area_uuid, but the model relationships used Laravel defaults, so service-area reloads did not include nested zones.

Validation

• php -l server/src/Models/ServiceArea.php server/src/Models/Zone.php server/tests/ServiceAreaZoneApiShapeTest.php
• php-cs-fixer dry run on touched PHP files
• git diff --check

Note: targeted Pest is blocked locally because server_vendor/pestphp/pest/vendor/autoload.php is missing in this checkout.