Fix Azure Ad Token Function's Return (#1460)

Identify returned "token" as string.

src/openai/lib/azure.py

@@ -519,7 +519,7 @@ async def _get_azure_ad_token(self) -> str | None:
                 raise ValueError(
                     f"Expected `azure_ad_token_provider` argument to return a string but it returned {token}",
                 )
-            return token
+            return str(token)
 
         return None