refactor(vertex): remove redundant isinstance check in load_auth (#1387)

Author: karpetrosyan

src/anthropic/lib/vertex/_auth.py

@@ -35,9 +35,6 @@ def load_auth(*, project_id: str | None) -> tuple[Credentials, str]:
     if not project_id:
         raise ValueError("Could not resolve project_id")
 
-    if not isinstance(project_id, str):
-        raise TypeError(f"Expected project_id to be a str but got {type(project_id)}")
-
     return credentials, project_id