Fix PHPStan errors - add user_id property annotation

- Added user_id property to FilamentFormUser model PHPDoc
- PHPStan now recognizes user_id property access

Author: scott grayson

src/Models/FilamentFormUser.php

@@ -13,6 +13,7 @@
 /**
  * @property array $entry
  * @property array|null $firstEntry
+ * @property int|null $user_id
  * @property-read array $key_value_entry
  * @property-read FilamentForm $filamentForm
  */