feat(core): add system store write conflict handling

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

shared/lib/SystemStore/SystemStoreException.php

@@ -9,6 +9,6 @@ use RuntimeException;
 /**
  * Raised when a tenant system store cannot be resolved or trusted.
  */
-final class SystemStoreException extends RuntimeException
+class SystemStoreException extends RuntimeException
 {
 }

shared/lib/SystemStore/WriteConflictException.php

@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace KTXF\SystemStore;
+
+/** Raised when an atomic write or delete precondition is not satisfied. */
+final class WriteConflictException extends SystemStoreException
+{
+}