scheduled_messages: Migrate to typed_endpoint.

Migrate `scheduled_message.py` to typed_endpoint.

Perform Json parsing in the endpoint itself instead of
in `recipient_parsing.py`.

zerver/lib/recipient_parsing.py

@@ -1,29 +1,17 @@
-import orjson
 from django.utils.translation import gettext as _
 
 from zerver.lib.exceptions import JsonableError
 
 
-def extract_stream_id(req_to: str) -> int:
+def extract_stream_id(req_to: int | list[int]) -> int:
     # Recipient should only be a single stream ID.
-    try:
-        stream_id = int(req_to)
-    except ValueError:
+    if isinstance(req_to, list):
         raise JsonableError(_("Invalid data type for channel ID"))
-    return stream_id
+    return req_to
 
 
-def extract_direct_message_recipient_ids(req_to: str) -> list[int]:
-    try:
-        user_ids = orjson.loads(req_to)
-    except orjson.JSONDecodeError:
-        user_ids = req_to
-
-    if not isinstance(user_ids, list):
+def extract_direct_message_recipient_ids(req_to: int | list[int]) -> list[int]:
+    if not isinstance(req_to, list):
         raise JsonableError(_("Invalid data type for recipients"))
 
-    for user_id in user_ids:
-        if not isinstance(user_id, int):
-            raise JsonableError(_("Recipient list may only contain user IDs"))
-
-    return list(set(user_ids))
+    return list(set(req_to))