fix(AssertIsTypeMethodCallRector): guard against non-string arg crashing isset on enum case (#674)

When `isType()` is called with an enum case argument, `ValueResolver::getValue()`
returns an enum object. Using it as an array key in `isset()` caused a fatal error:
"Cannot access offset of type SomeEnum in isset or empty".

Add `is_string()` guard before the array lookup so non-string args are safely skipped.

Fixes rectorphp/rector#9765

Co-authored-by: Claude <noreply@anthropic.com>

Author: TomasVotruba

rules-tests/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector/Fixture/skip_non_string_arg.php.inc

@@ -0,0 +1,20 @@
+<?php
+
+// skip: non-string argument (enum case) should not crash
+
+namespace Rector\PHPUnit\Tests\PHPUnit120\Rector\Class_\AssertIsTypeMethodCallRector\Fixture;
+
+use PHPUnit\Framework\TestCase;
+
+enum SkipNonStringArgEnum
+{
+    case ONE;
+}
+
+final class SkipNonStringArg extends TestCase
+{
+    public function testMethod(): void
+    {
+        $this->assertThat([], $this->isType(SkipNonStringArgEnum::ONE));
+    }
+}

rules/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector.php

@@ -114,6 +114,10 @@ public function refactor(Node $node): Node|null
         }
 
         $argValue = $this->valueResolver->getValue($arg);
+        if (! is_string($argValue)) {
+            return null;
+        }
+
         if (isset(self::IS_TYPE_VALUE_TO_METHOD[$argValue])) {
             if ($node instanceof MethodCall) {
                 return new MethodCall($node->var, self::IS_TYPE_VALUE_TO_METHOD[$argValue]);