Add PDF signature zone availability checks

Introduce `PDFSignatureZoneAvailable` service to check available PDF signature zones. Updated `WorkflowAddSignatureController` to use the new service. Added unit tests to verify the correctness of the functionality.

src/Bundle/ChillDocStoreBundle/Service/Signature/PDFSignatureZoneAvailable.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Chill\DocStoreBundle\Service\Signature;
+
+use Chill\DocStoreBundle\Service\StoredObjectManagerInterface;
+use Chill\MainBundle\Entity\Workflow\EntityWorkflow;
+use Chill\MainBundle\Entity\Workflow\EntityWorkflowSignatureStateEnum;
+use Chill\MainBundle\Entity\Workflow\EntityWorkflowStep;
+use Chill\MainBundle\Entity\Workflow\EntityWorkflowStepSignature;
+use Chill\MainBundle\Workflow\EntityWorkflowManager;
+
+class PDFSignatureZoneAvailable
+{
+    public function __construct(
+        private readonly EntityWorkflowManager $entityWorkflowManager,
+        private readonly PDFSignatureZoneParser $pdfSignatureZoneParser,
+        private readonly StoredObjectManagerInterface $storedObjectManager,
+    ) {}
+
+    /**
+     * @return list<PDFSignatureZone>
+     */
+    public function getAvailableSignatureZones(EntityWorkflow $entityWorkflow): array
+    {
+        $storedObject = $this->entityWorkflowManager->getAssociatedStoredObject($entityWorkflow);
+
+        if (null === $storedObject) {
+            throw new \RuntimeException('No stored object found');
+        }
+
+        if ('application/pdf' !== $storedObject->getType()) {
+            throw new \RuntimeException('Only PDF documents are supported');
+        }
+
+        $zones = $this->pdfSignatureZoneParser->findSignatureZones($this->storedObjectManager->read($storedObject));
+        $signatureZonesIndexes = array_map(
+            fn (EntityWorkflowStepSignature $step) => $step->getZoneSignatureIndex(),
+            $this->collectSignaturesInUse($entityWorkflow)
+        );
+
+        return array_filter($zones, fn (PDFSignatureZone $zone) => !in_array($zone->index, $signatureZonesIndexes, true));
+    }
+
+    /**
+     * @return list<EntityWorkflowStepSignature>
+     */
+    private function collectSignaturesInUse(EntityWorkflow $entityWorkflow): array
+    {
+        return array_reduce($entityWorkflow->getSteps()->toArray(), function (array $result, EntityWorkflowStep $step) {
+            $current = [...$result];
+            foreach ($step->getSignatures() as $signature) {
+                if (EntityWorkflowSignatureStateEnum::SIGNED === $signature->getState()) {
+                    $current[] = $signature;
+                }
+            }
+
+            return $current;
+        }, []);
+    }
+}