core[minor],community[minor]: Upgrade all @root_validator() to @pre_init (#23841)

This PR introduces a @pre_init decorator that's a @root_validator(pre=True) but with all the defaults populated!

libs/community/langchain_community/chat_models/jinachat.py

@@ -45,6 +45,7 @@ from langchain_core.utils import (
     convert_to_secret_str,
     get_from_dict_or_env,
     get_pydantic_field_names,
+    pre_init,
 )
 from tenacity import (
     before_sleep_log,
@@ -218,7 +219,7 @@ class JinaChat(BaseChatModel):
         values["model_kwargs"] = extra
         return values
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         values["jinachat_api_key"] = convert_to_secret_str(