Add function existence test in ClampTest

This commit adds a new test case in ClampTest.php to check if the 'clamp' function exists. This helps to catch errors caused by the function being undefined or removed inadvertently.

tests/ClampTest.php

@@ -37,4 +37,8 @@ public function testClampMinMax($value, $min, $max, $expected): void
     {
         $this->assertSame($expected, clampMinMax($value, $min, $max));
     }
+
+    public function testIfFunctionExists() {
+        $this->assertTrue(function_exists('clamp'), 'Function clamp does not exist');
+    }
 }