Improve code comments and comparison consistency

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

Author: Copilot

src/agent/src/Toolbox/StreamResult.php

@@ -39,9 +39,10 @@ public function getContent(): \Generator
                 }
 
                 $content = $innerResult->getContent();
-                // Strings are iterable in PHP but should be yielded as single values
-                // null should also be yielded as-is
-                if (is_string($content) || null === $content) {
+                // Strings are iterable in PHP but yield from would iterate character-by-character.
+                // We need to yield the complete string as a single value to preserve streaming behavior.
+                // null should also be yielded as-is.
+                if (is_string($content) || $content === null) {
                     yield $content;
                 } else {
                     yield from $content;