FlowState: add Status, thread invoker Task, simplify resume/interrupt

[stidsborg]

Summary

Scaffolding for centralised suspension via FlowsManager. No behaviour change yet — nothing calls FlowState.Suspend() in this PR.

• FlowState constructor now takes a completion Task and hooks a ContinueWith that flips Status to Completed when the invoker's TaskCompletionSource resolves. Setter guards Completed as terminal.
• Waiting() returns Subflows == WaitingSubflows.
• TryResumeSubflow() (returning bool) becomes ResumeSubflow() returning ForeverTask.Instance when suspended or Task.CompletedTask otherwise. Call site in QueueManager.Subscribe collapses to a single await.
• Interrupt() no longer zeros WaitingSubflows: that desynchronised the counter — subsequent ResumeSubflow calls drove it negative, which silently prevented future Suspend() from ever triggering. Now it just delegates to QueueManager.Interrupt() when not suspended.
• Invoker creates its TaskCompletionSource before PrepareFor[Re]Invocation and threads tcs.Task through CreateFlowState, so FlowState can observe the invocation's terminal state directly.

Test files that hand-roll FlowState/CreateFlowState (PrintEffectsTests, EffectTests, MessagesSubscriptionTests) now pass ForeverTask.Instance for the completion Task.

Follow-ups (not in this PR)

• Implement CheckForSuspension loop in FlowsManager and start it from FunctionsRegistry.
• Make Suspend() fault live QueueManager subscriptions with SuspendInvocationException so the user-function task exits and the existing PersistResult path runs.
• Decouple MessagesDefaultMaxWaitForCompletion from per-subscription auto-suspension in QueueManager.Subscribe.

Test plan

• In-memory suite: 451/451 pass
• PostgreSQL suite: 377/377 pass
• SqlServer / MariaDB: single-test runs pass for the central test paths; full-suite hang on PingPongMessagesCanBeExchangedMultipleTimes is pre-existing (reproduced on main baseline without this branch)