fix: treat duplicate reminder acks as idempotent no-ops

[Aaronontheweb]

What

Fixes a false reminder.settlement.failed / NotFound over-alert when the same reminder occurrence is acked more than once.

Root cause

ReminderManagerActor treated any non-Success AckAsync response (and any Error or NotFound NackAsync response) as a settlement failure, emitting reminder.settlement.failed. But Aaron.Akka.Reminders returns NotFound when the occurrence is no longer awaiting ack — i.e. a duplicate ack of an already-settled occurrence. That is benign and idempotent; the work already completed on the first ack.

Only Error represents a genuine failure.

Changes

• SettleBlockedOccurrenceAsync (ack + nack paths): only Error emits failure; NotFound is a silent no-op
• SettleSuccessfulExecutionAsync: NotFound no longer short-circuits or alerts
• SettleFailedExecutionAsync: only Error nack emits failure
• Added regression test Duplicate_ack_of_already_settled_occurrence_does_not_emit_settlement_failed_alert

Validation

• New test fails without the fix, passes with it (verified via stash round-trip)
• New test stable across 8 consecutive runs (fixed a frozen-clock actor-name collision by advancing FakeTimeProvider before the second fire)
• Full Reminders test suite: 162/162 pass
• slopwatch analyze: no new violations
• Header check passes

[Aaronontheweb]

LGTM