[CodeQuality] Move AssertClassToThisAssertRector hook from Class_ to ClassMethod

[TomasVotruba]

The rule only ever rewrote Assert::assert*() calls inside class methods, so hooking on ClassMethod is more precise than hooking on the whole Class_ and re-traversing it.

Behavior is unchanged:

 final class SomeTest extends TestCase
 {
     public function testMe()
     {
-        Assert::assertSame(1, 1);
+        $this->assertSame(1, 1);
     }
 }

Static methods are now skipped with an early return instead of a DONT_TRAVERSE_CURRENT_AND_CHILDREN check inside the traversal callback:

public function refactor(Node $node): ?Node
{
    if ($node->isStatic()) {
        return null;
    }
    // ...
}

Static closures and arrow functions are still skipped during traversal.

The rule and its tests move from Rector/Class_/ to Rector/ClassMethod/ to match the hooked node type.