fix: update getModel() method signature for Filament v4 compatibility

- Fix method signature incompatibility in SolutionForest\FilamentTree\Actions\Action::getModel()
- Add missing parameter: bool $withDefault = true
- Change return type from string to ?string to match parent class
- Resolves PHP Fatal Error: "Declaration of getModel(): string must be compatible with Filament\Actions\Action::getModel(bool $withDefault = true): ?string"

Fixes #78

Author: cklei-carly

src/Actions/Action.php

@@ -63,7 +63,7 @@ public function getPluralModelLabel(): string
         return $this->getCustomPluralModelLabel() ?? $this->getTree()->getPluralModelLabel();
     }
 
-    public function getModel(bool $withDefault = true): string
+    public function getModel(bool $withDefault = true): ?string
     {
         return $this->getCustomModel() ?? $this->getLivewire()->getModel();
     }