Fix duplicate task generation

[ndr-ds]

Motivation

The task processor can call process_actions multiple times while a previous batch's tasks are still in-flight.
When there's a backlog (schedule.start far in the past), the deadline timer fires
immediately because the next event is also in the past. The last_requested_callbacks guard passes, and
next_actions reads stale on-chain state (schedule.start hasn't advanced yet), generating overlapping timestamps.
When both batches are eventually submitted as blocks, batch 2's stale timestamps don't
match the now-advanced schedule.start, causing:

contract.rs:115: assert_eq!(timed_data.timestamp, next_timestamp, "unexpected timestamp in posted data")

Proposal

Introduce an in_flight_apps: BTreeSet<ApplicationId> guard in the task processor. When a batch of tasks is spawned
for an application, the app is marked as in-flight and subsequent process_actions calls skip it. A new
TaskMessage enum replaces the raw (ApplicationId, TaskOutcome) channel, adding a BatchComplete variant that the
spawned task sends after all outcomes. On BatchComplete, the guard is cleared and process_actions is
re-triggered for that app so it reads freshly-updated on-chain state.

Test Plan

• CI
• I had a stuck worker with a big backlog. After rebuilding pm-app with this fix and restarting the worker with the new package version, I see no more panics, and it seems that the worker is actually making progress through the backlog:

[ma2bd]

can we remove let _ = ?

[ndr-ds]

Yes, my bad #5436