Interrupt-driven message fetch; merge FetchedMessages into QueueManager

[stidsborg]

Summary

• Interrupt-driven fetch. Replaces the per-flow FetchLoop polling task with an event-driven model. InterruptedWatchdog → FlowsManager.Interrupt → QueueManager.Interrupt → FetchAndNotify. Cuts a background Task.Run per flow and removes the MessagesPullFrequency latency floor on delivery.
• Per-subscription timeouts. Each Subscribe schedules its own Task.Delay with a CancellationTokenSource, cancelled when the message arrives. Replaces the centralized FireTimeouts scan. The continuation only runs OnlyOnRanToCompletion and guards on _subscriptions.Remove(...) to prevent overwriting a delivered message with a null capture.
• Merge FetchedMessages into QueueManager. The split was an artifact of the polling architecture. A single class better describes "the message queue for a single flow." MessageData is now QueueManager.MessageData (public nested record used by QueueClient's capture callback).
• Init ordering. _effect.RegisterQueueManager(this) and _flowState.QueueManager = this are now set after initialization completes, so the InterruptedWatchdog can't reach FetchAndNotify while idempotency-key state and delivered-position cleanup are still being set up.

Test plan

• All 451 in-memory tests pass locally (dotnet test ./Core/Cleipnir.ResilientFunctions.Tests --filter "FullyQualifiedName~InMemoryTests")
• Database-backed test suites (Postgres / MariaDB / SQL Server)