fix: require symfony/string explicitly (#62)

We are using it in the extension

Author: OskarStark

composer.json

@@ -14,7 +14,8 @@
         "php-llm/llm-chain": "^0.13",
         "symfony/config": "^6.4 || ^7.0",
         "symfony/dependency-injection": "^6.4 || ^7.0",
-        "symfony/framework-bundle": "^6.4 || ^7.0"
+        "symfony/framework-bundle": "^6.4 || ^7.0",
+        "symfony/string": "^6.4 || ^7.0"
     },
     "require-dev": {
         "php-cs-fixer/shim": "^3.65",