create-channel: Remove send_new_subscription_messages parameter.

Removes send_new_subscription_messages parameter from
`POST channel/create` endpoint as Notification Bot DMs
are never sent when users are subscribed as part of
creating a new channel.

Not considered a documentable API change, since the previous parameter
never had any effect, so this is effectively just a documentation and
error-handling bug fix, not an API change relevant to implementors.

zerver/openapi/zulip.yaml

@@ -23730,8 +23730,6 @@ paths:
                   example: true
                 folder_id:
                   $ref: "#/components/schemas/FolderId"
-                send_new_subscription_messages:
-                  $ref: "#/components/schemas/SendNewSubscriptionMessages"
                 topics_policy:
                   $ref: "#/components/schemas/TopicsPolicy"
                 history_public_to_subscribers:

zerver/tests/test_channel_creation.py

@@ -24,6 +24,7 @@ from zerver.models import (
     Message,
     NamedUserGroup,
     Realm,
+    Recipient,
     Stream,
     Subscription,
     UserMessage,
@@ -303,6 +304,18 @@ class TestCreateStreams(ZulipTestCase):
         self.assertEqual(stream.name, "testchannel")
         self.assertEqual(stream.description, "test channel")
 
+        # Confirm channel created notification message in channel events topic.
+        message = self.get_last_message()
+        self.assertEqual(message.recipient.type, Recipient.STREAM)
+        self.assertEqual(message.recipient.type_id, stream.id)
+        self.assertEqual(message.topic_name(), Realm.STREAM_EVENTS_NOTIFICATION_TOPIC_NAME)
+        self.assertEqual(message.sender_id, self.notification_bot(user_profile.realm).id)
+        expected_message_content = (
+            f"**Public** channel created by @_**{user_profile.full_name}|{user_profile.id}**. **Description:**\n"
+            "```` quote\ntest channel\n````"
+        )
+        self.assertEqual(message.content, expected_message_content)
+
         # Creating an existing channel should return an error.
         result = self.create_channel_via_post(user_profile, name="basketball")
         self.assert_json_error(result, "Channel 'basketball' already exists", status_code=409)

zerver/views/streams.py

@@ -738,7 +738,6 @@ def create_channel(
     is_default_stream: Json[bool] = False,
     message_retention_days: Json[str] | Json[int] = RETENTION_DEFAULT,
     name: Annotated[str, StringConstraints(strip_whitespace=True, min_length=1)],
-    send_new_subscription_messages: Json[bool] = True,
     subscribers: Json[list[int]],
     topics_policy: Json[TopicsPolicy] = None,
 ) -> HttpResponse:
@@ -825,18 +824,16 @@ def create_channel(
         new_subscribers,
         acting_user=user_profile,
     )
-    if (
-        send_new_subscription_messages
-        and len(new_subscribers) <= settings.MAX_BULK_NEW_SUBSCRIPTION_MESSAGES
-    ):
-        send_user_subscribed_and_new_channel_notifications(
-            user_profile=user_profile,
-            subscribers=new_subscribers,
-            new_subscriptions={str(user.id): [name] for user in new_subscribers},
-            id_to_user_profile={str(user.id): user for user in new_subscribers},
-            created_streams=[new_channel],
-            announce=announce,
-        )
+    # We never send DM notifications about newly created channels, so we only
+    # need to send the data for the new channel notification messages.
+    send_user_subscribed_and_new_channel_notifications(
+        user_profile=user_profile,
+        subscribers=set(),
+        new_subscriptions={},
+        id_to_user_profile={},
+        created_streams=[new_channel],
+        announce=announce,
+    )
 
     return json_success(
         request,