[Feature]: Runtime Notification Dispatch Overload

[eneshoxha]

Title: Add non-generic PublishAsync(INotification) overload for runtime-typed notifications

Labels: enhancement, mediator

Body:

Problem

IMediator.PublishAsync<TNotification> requires the notification type to be known at compile time. There is no runtime-dispatch overload like:

Task PublishAsync(INotification notification, CancellationToken cancellationToken = default);

This makes it impossible to publish notifications when the concrete type is determined at runtime (e.g., deserializing events from a message bus, dispatching domain events from an aggregate root's List<INotification>).

Commands and queries already have this pattern (SendCommandAsync(ICommand<TResult>) and SendQueryAsync(IQuery<TResult>) use reflection-based dispatch).

Proposed Solution

Add to IMediator:

Task PublishAsync(INotification notification, CancellationToken cancellationToken = default);

Implementation in Mediator.cs should follow the same reflection + ConcurrentDictionary<Type, MethodInfo> caching pattern used for commands and queries.