T6 - subscription types

[kubinio123]

Target branch: 2026-07-28-protocol-support

Add SubscriptionFilter, SubscriptionsListenParams, SubscriptionsListenResult, SubscriptionsAcknowledgedParams and the notification wrapper and schema tests.

Spec:

Subscriptions - Opening a Stream
Subscriptions - Notification Filter
Subscriptions - Acknowledgment
Schema - SubscriptionFilter
Schema - SubscriptionsListenResult

Conformance scenarios that must pass after this task: none.

Docs: no.