Add test for property with default value in set hook isset check

When a property has a default value (e.g. $bar = 1), it is always
initialized, so isset() in a set hook should still report an error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Rules/Variables/IssetRuleTest.php

@@ -531,7 +531,12 @@ public function testBug13473(): void
 	{
 		$this->treatPhpDocTypesAsCertain = true;
 
-		$this->analyse([__DIR__ . '/data/bug-13473.php'], []);
+		$this->analyse([__DIR__ . '/data/bug-13473.php'], [
+			[
+				'Property Bug13473\Bar::$bar (int) in isset() is not nullable.',
+				30,
+			],
+		]);
 	}
 
 	public function testBug14393(): void

tests/PHPStan/Rules/Variables/data/bug-13473.php

@@ -22,3 +22,20 @@ public function __construct(int $bar)
 }
 
 $foo = new Foo(10);
+
+class Bar {
+    private(set) int $bar = 1 {
+        get => $this->bar;
+        set(int $bar) {
+            if (isset($this->bar)) {
+                throw new \Exception('bar is set');
+            }
+            $this->bar = $bar;
+        }
+    }
+
+    public function __construct(int $bar)
+    {
+        $this->bar = $bar;
+    }
+}