fix: report an unresolvable hotel instead of failing on it

src/Validator/Constraints/ResolvableHotel.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Validator\Constraints;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * The chosen hotel must still resolve against MyE&P's base data.
+ *
+ * Everything a destination records about its house is derived from that lookup - name, code,
+ * cost unit, country - so an id that no longer resolves cannot produce a complete row. It used
+ * to reach App\Controller\Administrative\System\Destination\DestinationTrait::updateEntity()
+ * and fail there on a null, which is a 500 on a form submit rather than a message on a field.
+ */
+#[\Attribute]
+class ResolvableHotel extends Constraint
+{
+    public string $unknownMessage = 'Dieser Vertragspartner ist nicht mehr verfügbar';
+
+    public string $unavailableMessage = 'Die Stammdaten sind gerade nicht erreichbar – bitte später erneut versuchen';
+}

src/Validator/Constraints/ResolvableHotelValidator.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Validator\Constraints;
+
+use App\BusProNet\DataProvider\HotelDataProvider;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class ResolvableHotelValidator extends ConstraintValidator
+{
+    public function __construct(private readonly HotelDataProvider $hotelDataProvider)
+    {
+    }
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        if (false === $constraint instanceof ResolvableHotel) {
+            return;
+        }
+
+        // Whether a hotel was chosen at all is NotBlank's business, not this one's.
+        if (null === $value || '' === $value) {
+            return;
+        }
+
+        // An empty list is MyE&P being unreachable rather than a bad choice: StaleWhileErrorCache
+        // answers [] instead of throwing, so this is the only place the two are still
+        // distinguishable. Reporting an outage as an invalid selection sends the operator looking
+        // for a different answer when the one they gave was right.
+        if ([] === $hotels = $this->hotelDataProvider->getAll()) {
+            $this->context->buildViolation($constraint->unavailableMessage)->addViolation();
+
+            return;
+        }
+
+        if (false === isset($hotels[(int) $value])) {
+            $this->context->buildViolation($constraint->unknownMessage)->addViolation();
+        }
+    }
+}