v3/feature/215: Pluggable Notification Publish Strategies

[eneshoxha]

Changes

New files:

• Notifications/INotificationPublishStrategy.cs — Interface with a single PublishAsync(IEnumerable<Func>, CancellationToken) method. Strategies receive pre-built handler pipeline delegates and decide how to execute them.
• Notifications/ParallelNotificationStrategy.cs — Default strategy. Executes all handlers in parallel via Task.WhenAll (preserves current behavior).
• Notifications/SequentialNotificationStrategy.cs — Executes handlers one at a time in registration order. Respects cancellation between handlers.
• Notifications/StopOnFirstFailureNotificationStrategy.cs — Sequential execution that stops immediately on first exception, skipping remaining handlers.

Modified files:

• MediatorOptions.cs — Added NotificationPublishStrategyType (internal) and UseNotificationPublishStrategy() fluent method. Default is ParallelNotificationStrategy.
• ServiceCollectionExtensions.cs — Registers INotificationPublishStrategy as singleton from the configured type.
• Mediator.cs — Refactored PublishAsync to build Func[] delegates and delegate execution to INotificationPublishStrategy. Falls back to parallel when no strategy is registered (backward compatible for manual DI).

Tests — NotificationPublishStrategyTests.cs (13 tests):

• Default strategy is Parallel
• UseNotificationPublishStrategy DI registration
• Fluent API returns options
• Parallel: all handlers execute, exceptions propagate
• Sequential: order preserved, stops on exception, respects cancellation
• StopOnFirstFailure: all succeed case, stops on first exception
• Integration with AddCortexMediator
• Fallback when no strategy registered
• Custom strategy support (reverse order strategy)