[CHA-5933] feat: add scheduled messages support

[nijeesh-stream]

TLDR

Adds server-side support for Chat scheduled messages: schedule a message for a user, then edit it, cancel it, send it right away, or list a user's scheduled messages. Backend endpoints are behind a per-app flag and not yet released; merge after backend CHA-5924 ships. Risk is low: the change adds new methods and nullable fields. One send-path tweak applies only when ScheduledAt is set (details below).

Linear: https://linear.app/stream/issue/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)
• Commit message fits conventional commits. Important for CHANGELOG generation.

Description of the pull request

New IMessageClient methods, next to the reminder methods:

Method	API
ScheduleMessageAsync(type, id, msg, userId, scheduledAt, options), or SendMessageAsync with SendMessageOptions.ScheduledAt	POST /channels/{type}/{id}/message with scheduled_at; the response's new GenericMessageResponse.ScheduledMessage is set
UpdateScheduledMessageAsync(id, UpdateScheduledMessageRequest)	PATCH /scheduled_messages/{id}
DeleteScheduledMessageAsync(id)	DELETE /scheduled_messages/{id}
SendScheduledMessageNowAsync(id)	POST /scheduled_messages/{id}/send_now
QueryScheduledMessagesAsync(QueryScheduledMessagesRequest)	POST /scheduled_messages/query (UserId, Filter.Cid, Filter.Status, Sort, Limit, Next)

Models (src/Models/ScheduledMessage.cs): ScheduledMessage, ScheduledMessageContent (custom fields via GetData), ScheduledMessageLocation, ScheduledMessageChannel, the request and response types, and the ScheduledMessageStatus / ScheduledMessageFailureReason string constants. EventResponse gains ScheduledMessage and MessageId, so WebhookHelpers.ParseEvent exposes the scheduled_message.sent / .failed webhooks.

Send-path note: SendMessageAsync always sends is_pending_message (false by default), and scheduled sends do the same, exactly like a live send. An explicit false overrides the channel type's mark_messages_pending setting, so a scheduled message into such a channel is sent normally instead of failing with pending_not_supported. Requires a backend with GetStream/chat#18727, which accepts is_pending_message: false with scheduled_at (older backends answered 400 scheduled_at cannot be combined with pending). Merge and release only after that is deployed everywhere. IsPendingMessage = true with ScheduledAt is still rejected by the API.

Docs: new docs/messages/scheduled_messages.md, plus the webhook events in docs/webhooks/webhook_events.md. CHANGELOG is left to the release tooling.

Not added: include_scheduled_messages on GetOrCreateChannel (the backend ignores it for server-side calls), app settings (the chat_scheduled_messages_* flags aren't exposed through the app settings API), and a permission constant (Resource only lists legacy v1 permissions).

Tests

• tests/ScheduledMessageRequestTests.cs: checks request and response shapes, and webhook parsing, against a stubbed HttpMessageHandler. It needs no credentials and always runs.
• MessageClientTests: live round trip (schedule, query, update, send now, delete). It calls Assert.Ignore when the app has the feature off or the backend ignores scheduled_at, so CI stays green until the flag is on for the CI app.
• Local results: the live tests pass (5/5) against a local backend built from the backend branch feature/cha-5924-scheduled-messages-channel-state, using an app with the flag on. They show as ignored against an app with the flag off. The net6.0 build has no new warnings.

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