refactor(Tool): streamline handler resolution in Tool class

- Introduced a new method `resolveHandler()` to encapsulate the logic for determining the callable handler for the tool.
- Simplified the `handle()` method by delegating handler resolution to `resolveHandler()`, improving readability and maintainability.
- Enhanced error handling for cases where no valid handler is defined, ensuring consistent behavior across invokable subclasses.

Author: vinitkadam03

src/Tool.php

@@ -266,19 +266,7 @@ public function failedHandler(): null|false|Closure
     public function handle(...$args): string|ToolOutput|ToolError
     {
         try {
-            $fn = $this->fn ?? (method_exists($this, '__invoke') ? $this : null);
-
-            if ($fn === null) {
-                throw new PrismException("Tool handler not defined for tool: {$this->name}");
-            }
-
-            // After ProcessDriver deserialization, $fn may become a
-            // SerializableClosure\Serializers\Native whose __invoke doesn't
-            // forward PHP 8 named arguments. Unwrap via getClosure() to
-            // recover the real Closure so named-arg spreading works.
-            $callable = is_object($fn) && method_exists($fn, 'getClosure')
-                ? $fn->getClosure()
-                : $fn;
+            $callable = $this->resolveHandler();
 
             $value = call_user_func($callable, ...$args);
 
@@ -299,6 +287,35 @@ public function handle(...$args): string|ToolOutput|ToolError
         }
     }
 
+    /**
+     * Resolve the callable handler for this tool.
+     *
+     * Priority: explicit $fn > invokable subclass (__invoke) > error.
+     * Also unwraps SerializableClosure wrappers that break named arguments.
+     */
+    protected function resolveHandler(): callable
+    {
+        $fn = $this->fn;
+
+        if ($fn === null && method_exists($this, '__invoke')) {
+            $fn = $this;
+        }
+
+        if ($fn === null) {
+            throw new PrismException("Tool handler not defined for tool: {$this->name}");
+        }
+
+        // After ProcessDriver deserialization, $fn may become a
+        // SerializableClosure\Serializers\Native whose __invoke doesn't
+        // forward PHP 8 named arguments. Unwrap via getClosure() to
+        // recover the real Closure so named-arg spreading works.
+        if (is_object($fn) && method_exists($fn, 'getClosure')) {
+            return $fn->getClosure();
+        }
+
+        return $fn;
+    }
+
     protected function shouldHandleErrors(): bool
     {
         return $this->failedHandler !== false;