[CHA-5933] feat: support scheduled messages

[nijeesh-stream]

TLDR

Backends can now schedule a chat message to be sent later, on behalf of any user, and then list, edit, cancel or send early those scheduled messages. Message.send(...).scheduledAt(date) stores the message instead of sending it; the new ScheduledMessage class covers the rest. Risk is low: the change is additive (one new optional request field, two new optional response fields, one new model and service). The backend endpoints are behind a per-app flag and not yet released, so merge after backend CHA-5924 ships.

Ticket

CHA-5933

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Backend endpoints are behind a per-app flag and not yet released; merge after backend CHA-5924 ships.

API surface

Call	Endpoint
Message.send(type, id).scheduledAt(date)	POST /channels/{type}/{id}/message with scheduled_at; response gains scheduled_message
ScheduledMessage.query().userId(..).filter(..).sort(..).limit(..).next(..)	POST /scheduled_messages/query (user_id is required server-side; filter cid and status; only scheduled_at ascending; limit 1-100, default 20; cursor next)
ScheduledMessage.update(id).message(..).scheduledAt(..)	PATCH /scheduled_messages/{id} (either or both)
ScheduledMessage.delete(id)	DELETE /scheduled_messages/{id} (cancel, or remove a failed one)
ScheduledMessage.sendNow(id)	POST /scheduled_messages/{id}/send_now

• ScheduledMessage mirrors the backend ScheduledMessageResponse: id, cid, user, message (content without an ID; mentioned_users holds user IDs), scheduled_at, status, failure_reason, created_at, updated_at, and channel (query results only). Status values are exposed as STATUS_* constants.
• Event gains scheduledMessage and messageId (the sent message's ID on scheduled_message.sent).
• New doc page docs/messages/scheduled_messages.md.

Not added, on purpose:

• include_scheduled_messages on GetOrCreateChannel: the backend marks it ignore_if_server_side, so it does nothing for a server SDK.
• App settings (chat_scheduled_messages_enabled, max lead time): not exposed through the app settings API.
• CHANGELOG.md: generated at release time.

How to verify

1. ./gradlew test --tests '*ScheduledMessageTest*' passes (8 serialization tests, no API credentials needed).
2. ./gradlew spotlessCheck javadoc passes.
3. ScheduledMessageIntegrationTest (schedule, query, update, delete, send now against a real app) runs only with STREAM_SCHEDULED_MESSAGES_ENABLED=true, because the feature is behind a per-app flag that is off on the CI app. It has not been run yet: no app with the feature is available until the backend ships.

🤖 Generated with Claude Code