fix(session): initialize data lazily in save() to prevent uninitialized property access (#252)

src/Server/Session/Session.php

@@ -51,7 +51,7 @@ class Session implements SessionInterface
 
     public function save(): bool
     {
-        return $this->store->write($this->id, json_encode($this->data, \JSON_THROW_ON_ERROR));
+        return $this->store->write($this->id, json_encode($this->readData(), \JSON_THROW_ON_ERROR));
     }
 
     public function get(string $key, mixed $default = null): mixed