[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 only adds new methods, and existing calls are untouched.

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)

Description of the pull request

New methods, mirroring the reminders / drafts layout:

Method	API
Channel#schedule_message(message, user_id, scheduled_at, **options)	POST /channels/{type}/{id}/message with scheduled_at; the response carries scheduled_message
Client#update_scheduled_message(id, message:, scheduled_at:)	PATCH /scheduled_messages/{id}
Client#delete_scheduled_message(id)	DELETE /scheduled_messages/{id}
Client#send_scheduled_message_now(id)	POST /scheduled_messages/{id}/send_now
Client#query_scheduled_messages(user_id, filter:, sort:, **options)	POST /scheduled_messages/query (filter.cid, filter.status, limit, next)

scheduled_at accepts DateTime, Time or an RFC 3339 string, normalized like remind_at. Docs: new docs/messages/scheduled_messages.md, plus the scheduled_message.sent / scheduled_message.failed 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 (the SDK has no list of v2 permission actions).

Tests

• spec/scheduled_messages_spec.rb: checks the request shape against a stubbed Faraday adapter. It needs no credentials and always runs.
• spec/client_spec.rb → scheduled messages: live round trip (schedule, query, update, send now, delete). It skips when the app has the feature off (scheduled messages are not enabled) or the backend ignores scheduled_at, so CI stays green until the flag is on for the CI app.
• Local results: the live specs 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 skip as expected against an app with the flag off. Rubocop and srb tc are clean (no new Sorbet errors).

🤖 Generated with Claude Code

[nijeesh-stream]

CI note: the 💎 Ruby 3.4 failure is unrelated to this PR. Its lint step runs srb tc, which reports 2 errors from the vendored sorbet/rbi/hidden-definitions/hidden.rbi:9107 (extend ::URI::Escape, which no longer resolves). The docs-only PR #220 fails the same way, and srb tc on master shows the same 2 errors locally. Ruby 3.0 and 3.1 pass. The live scheduled-message specs are pending there because the CI app's backend doesn't return scheduled_message yet.

🤖 Generated with Claude Code