[Feature]: Resilience Pipeline Behavior (Polly Integration)

[eneshoxha]

Title: Add resilience pipeline behaviors with Polly v8 / Microsoft.Extensions.Resilience integration

Labels: enhancement, mediator, new-package

Body:

Problem

There are no built-in retry, circuit-breaker, or timeout pipeline behaviors. In event-driven architectures (especially with Cortex.Streams and Kafka), transient failures are common and handlers that call external services need resilience patterns.

Proposed Solution

Create a Cortex.Mediator.Behaviors.Resilience package:

• ResilienceCommandBehavior<TCommand, TResult> -- wraps command execution with a Polly ResiliencePipeline
• ResilienceQueryBehavior<TQuery, TResult> -- same for queries
• Support per-handler resilience via IResilientRequest interface or [Resilient] attribute
• Global default resilience pipeline via options

services.AddMediatorResilience(options =>
{
    options.DefaultPipeline = new ResiliencePipelineBuilder()
        .AddRetry(new RetryStrategyOptions { MaxRetryAttempts = 3 })
        .AddTimeout(TimeSpan.FromSeconds(10))
        .Build();
});

Dependencies

• Microsoft.Extensions.Resilience (which wraps Polly v8)