Simplify content type check for better maintainability

Co-authored-by: OskarStark <995707+OskarStark@users.noreply.github.com>

Author: Copilot

src/agent/src/Toolbox/StreamResult.php

@@ -39,10 +39,12 @@ public function getContent(): \Generator
                 }
 
                 $content = $innerResult->getContent();
-                if ($content instanceof \Generator || $content instanceof \Iterator || (is_array($content))) {
-                    yield from $content;
-                } else {
+                // Strings are iterable in PHP but should be yielded as single values
+                // null should also be yielded as-is
+                if (is_string($content) || null === $content) {
                     yield $content;
+                } else {
+                    yield from $content;
                 }
 
                 break;