Implement SignalR subscription gateway for real-time event streaming

[qodo-free-for-open-source-projects]

Description

• Enable real-time event streaming from Eventuous subscriptions to connected clients via SignalR
• Bridge the existing Gateway pattern (GatewayHandler + BaseProducer) with SignalR hubs
• Provide both server-side subscription management and client-side typed subscription API
• Support position-based deduplication and automatic reconnection with state recovery

Deliverables

• Server Gateway: SubscriptionGateway<THub> managing per-connection subscription lifecycle with SignalRProducer and SignalRTransform
• Server Hub: SignalRSubscriptionHub exposing SubscribeToStream and UnsubscribeFromStream methods
• Client Streaming: SignalRSubscriptionClient providing IAsyncEnumerable<StreamEventEnvelope> with auto-reconnect
• Typed Handlers: TypedStreamSubscription with fluent On<T> API and optional OpenTelemetry tracing
• Wire Contracts: Shared StreamEventEnvelope and StreamSubscriptionError via compile-time source linking
• Unit Tests: 12 tests covering producer, transform, gateway lifecycle, and typed subscription

[linear]

EVE-228

[alexeyzimarev]

#526