Vendored the notification channel api and exposed the channel RPCs.

[moetemp]

This PR brings the notification channel api into Core and exposes the channel RPCs to language SDKs.

What changed?

• The vendored api tree gets the channel api from the two api PRs, the channel service (Added the notification channel service. api#2) and the workflow as a listener (Added the workflow listener of a notification channel. api#3): temporal.api.notification.v1, the five channel RPCs and their messages, the subscribe and unsubscribe commands and events, notifications on the Workflow Task scheduled event, and channel_subscriptions on describe.
• The protos crate's type maps name the two new commands and events. Neither event is ignorable.
• NotificationsReceived is a new activation job. The Rust SDK and the Rust workflow runtime have no channel API yet, so they ignore it.
• notify_channel, register_channel_listener, unregister_channel_listener, poll_channel and describe_channel are on the Core client and in the C bridge dispatch.
• The serde derive list and the payload-limit table know the notification package. Notification.metadata isn't validated against the blob limit, since the server bounds it with its own notification size limit.

Part of AI-198 (epic AI-37).

Why?

Language SDKs reach the server through Core, so a channel API in Python needs these RPCs on the Core client first. The generated Python RPC table doesn't compile without them. The job has to exist before Core can deliver notifications, and that comes in the next PR together with the commands and their machines. This PR has no commands, so it can't send anything Core can't handle yet.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

The workspace builds, cargo lint, cargo test-lint and cargo fmt --check are clean, and every commit builds on its own. The protos, common, client, SDK and workflow crates' unit tests pass. The vendored files are byte-identical to the api PR.

[moetemp]

Paused per the 2026-10-05 design review (the decision record is on the proposal page, section 14). Resumes in phase 2 with the CHASM notification work. The branch stays as a pin.