Bug Fix: TemplateIterator not discovering templates with @! namespace prefix

The TemplateIterator (used by the cache warmer) was not discovering templates registered with the @! namespace prefix, even though TwigExtension registers them with the Twig loader. This caused incomplete cache warming for templates referenced as @!BundleName/template.twig during cache:warmup, requiring additional compilation at runtime.

Example:

// Both are valid in Twig:
$this->render('@Foo/index.html.twig');   // Uses override if exists, otherwise bundle template
$this->render('@!Foo/index.html.twig');  // Always uses original bundle template

Before: Only @Foo/index.html.twig was compiled during cache:warmup
After: Both @Foo/index.html.twig and @!Foo/index.html.twig are compiled during cache:warmup

The fix adds one line to match the behavior of TwigExtension::load():

$templates[] = $this->findTemplatesInDirectory($bundleTemplatesDir, '!'.$name);

Tests: Updated TemplateIteratorTest to verify templates with the @! prefix are included.
Verified: Fix was also tested on the original bug reproducer: https://github.com/mbessolov/twig-override-cache-reproducer