Update grant_types to allow string values

Author: automaton82

src/mcp/shared/auth.py

@@ -47,9 +47,9 @@ class OAuthClientMetadata(BaseModel):
     # ie: we do not support client_secret_basic
     token_endpoint_auth_method: Literal["none", "client_secret_post"] = "client_secret_post"
     # grant_types: this implementation only supports authorization_code & refresh_token
-    grant_types: list[Literal["authorization_code", "refresh_token"]] = [
+    grant_types: list[Union[Literal["authorization_code", "refresh_token"], str]] = [
         "authorization_code",
-        "refresh_token",
+        "refresh_token"
     ]
     # this implementation only supports code; ie: it does not support implicit grants
     response_types: list[Literal["code"]] = ["code"]