fix: normalize tenant to string to match database round-trip type

setTenant() accepts int|string|null but PDO returns tenant as a string
after storage. The strict !== comparisons in getCollection() then fail
because (int)1 !== "1", causing "Collection not found" errors. The
Sequence validator also rejects integer tenant values since $tenant
has type VAR_ID which requires strings.

Cast tenant to string in Adapter::setTenant() and in the
tenantPerDocument document paths where tenant is set directly on
the document.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: loks0n

src/Database/Adapter.php

@@ -225,7 +225,7 @@ public function getSharedTables(): bool
      */
     public function setTenant(int|string|null $tenant): bool
     {
-        $this->tenant = $tenant;
+        $this->tenant = $tenant !== null ? (string) $tenant : null;
 
         return true;
     }

src/Database/Database.php

@@ -5503,12 +5503,16 @@ public function createDocument(string $collection, Document $document): Document
 
         if ($this->adapter->getSharedTables()) {
             if ($this->adapter->getTenantPerDocument()) {
+                $tenant = $document->getTenant();
                 if (
                     $collection->getId() !== static::METADATA
-                    && $document->getTenant() === null
+                    && $tenant === null
                 ) {
                     throw new DatabaseException('Missing tenant. Tenant must be set when tenant per document is enabled.');
                 }
+                if ($tenant !== null) {
+                    $document->setAttribute('$tenant', (string) $tenant);
+                }
             } else {
                 $document->setAttribute('$tenant', $this->adapter->getTenant());
             }
@@ -5621,9 +5625,11 @@ public function createDocuments(
 
             if ($this->adapter->getSharedTables()) {
                 if ($this->adapter->getTenantPerDocument()) {
-                    if ($document->getTenant() === null) {
+                    $tenant = $document->getTenant();
+                    if ($tenant === null) {
                         throw new DatabaseException('Missing tenant. Tenant must be set when tenant per document is enabled.');
                     }
+                    $document->setAttribute('$tenant', (string) $tenant);
                 } else {
                     $document->setAttribute('$tenant', $this->adapter->getTenant());
                 }
@@ -7198,9 +7204,11 @@ public function upsertDocumentsWithIncrease(
 
             if ($this->adapter->getSharedTables()) {
                 if ($this->adapter->getTenantPerDocument()) {
-                    if ($document->getTenant() === null) {
+                    $tenant = $document->getTenant();
+                    if ($tenant === null) {
                         throw new DatabaseException('Missing tenant. Tenant must be set when tenant per document is enabled.');
                     }
+                    $document->setAttribute('$tenant', (string) $tenant);
                     if (!$old->isEmpty() && $old->getTenant() !== $document->getTenant()) {
                         throw new DatabaseException('Tenant cannot be changed.');
                     }