PROCESSINGS: EventV2ProcessingService - Loop Detection

[cjdutoit]

Adds loop detection logic to the EventV2 processing service.

• Added RetrieveEventV2CountBySignatureAsync(EventV2, CancellationToken) pass-through delegating to the EventV2 foundation, with full exception suite (dependency validation, dependency, timeout, cancellation, service)
• Added IsLoopDetectedAsync(EventV2, CancellationToken) — reads LoopDetection config, returns false when disabled (fail-open), returns count > config.Threshold otherwise; call order enforced via MockSequence in the logic test
• Injected IConfigurationBroker into EventV2ProcessingService (broker — excluded from the 2–3 service-dependency count)
• Added ReturningIntFunction and ReturningBoolFunction delegate types with corresponding TryCatch overloads in the exceptions partial

closes #547