test: cover a deferred save crossing ClineProvider.abandonSubtask (cross-layer regression) #2150

Context

PR #1496 fixes #1021: a fire-and-forget Task#saveClineMessages() could run updateTaskHistory() after ClineProvider#abandonSubtask()'s atomicUpdatePair() cleared the child's parentTaskId/rootTaskId, silently reattaching the severed link. The fix adds an abandoned guard in Task#saveClineMessages() and unit coverage in src/core/task/__tests__/Task.spec.ts.

CodeRabbit's review asked for a cross-layer integration regression test: seed live parent/root ids, defer the message save across a real ClineProvider.abandonSubtask() call, and assert the persisted child history stays detached while the parent becomes active.

Why it is a follow-up

The two layers are currently covered separately:

• src/core/task/__tests__/Task.spec.ts — the abandoned-save guard, including a save that is already in flight when requestAbort(true) runs.
• src/__tests__/abandonSubtask.spec.ts — the provider-side transitions that clear the persisted links (atomicUpdatePair, parent back to active).

Exercising them together needs a real Task instance with deferred persistence inside the provider spec: the task constructor requires a full provider stub, and taskMetadata/updateTaskHistory/message persistence have to be controlled so the save can be paused across abandonSubtask(). That is a test-harness change rather than a behavior change, so it is split out to keep #1496 reviewable.

Acceptance criteria

• A provider-level test seeds a child history item with parentTaskId/rootTaskId and a delegated parent.
• A real Task save is deferred (message persistence pending) while abandonSubtask() runs.
• After the deferred save resolves, updateTaskHistory() was not applied for the child (no reattach) and the parent is active with awaitingChildId/delegatedToId cleared.
• The test fails if the this.abandoned guard is removed from Task#saveClineMessages().

References

• PR fix(task): guard saveClineMessages against abandoned tasks (fixes #1021) #1496 (the fix and unit tests)
• fix(task): guard saveClineMessages against abandoned tasks to prevent race in abandonSubtask #1021 (abandoned-subtask save race)