test: add SQLi rejection and edge-case tests for gc() type safety

Author: gr8man

tests/system/Session/Handlers/Database/AbstractHandlerTestCase.php

@@ -126,10 +126,11 @@ public function testGC(): void
 
     public function testGCRejectsNonInt(): void
     {
-        $handler = $this->getInstance();
+        $handler   = $this->getInstance();
+        $malicious = 'malicious; DROP TABLE sessions; --';
 
         $this->expectException(TypeError::class);
-        $handler->gc('malicious; DROP TABLE sessions; --'); // @phpstan-ignore argument.type
+        $handler->gc($malicious); // @phpstan-ignore argument.type
     }
 
     public function testGCWithZero(): void