Native: abandoned remote activities must outlive their run (cancellation_policy abandon) #971

Split from #958 / #969 after Security's review (#969 (comment)).

Today: native refuses an explicit cancellation_policy: abandon on schedule_activity with 409 activity_cancellation_policy_not_supported and unavailable: ["installed_runtime_activity_policy"] (#969). try_cancel and wait_cancellation_completed are supported.

Why: under Core 2.15.12, an abandoned remote activity keeps running after its parent run reaches its cooperative-cancelled terminal state, until it finishes or its own schedule_to_close deadline expires (DefaultActivityTaskBridge::poll, ActivityAbandonment::allows). Native ties activity work to an active parent:

• both activity candidate queries (TASK_CANDIDATES_SQL and the cross-kind poll) require the parent pending, running or waiting;
• complete_activity (and failure, heartbeat and lease paths, to be audited) go through active_run and refuse run_closed.

An abandoned task would be stranded once the run closes.

Needed: a narrow, fail-closed post-run authority. It requires the exact ActivityScheduled abandon snapshot, its schedule_to_close_deadline_at, and the run's matching canonical cooperative-cancelled close. Ordinary activities keep today's refusals. Cases from Security's review:

1. an unclaimed ready abandoned activity stays pollable after the parent canonically cancels;
2. a previously leased abandoned activity can complete or fail after that close, and records its terminal history;
3. ordinary activities, and runs closed as completed, failed or terminated (or with corrupt history), stay refused;
4. expiry still closes the activity at its original schedule-to-close deadline.

Then admit abandon again: the policy, its finite schedule_to_close_timeout (422), the snapshot deadline, and delivery without cancelling are already in #969's history.