Calling flaky HTTP APIs in .NET: IHttpClientFactory with Polly v8 standard resilience handler

// ======================================================================
// Q: How do you call a flaky third-party HTTP API?
//
// - IHttpClientFactory: avoids the socket exhaustion of new HttpClient() on
//   every call.
// - Retry only on transient errors (5xx, 408, 429) and only on idempotent calls.
//   A POST without an idempotency key is not retried blindly.
// - Circuit breaker, so we don't overwhelm an ERP that is already struggling.
// - A typed client with our own DTOs = anti-corruption layer: the ERP's model
//   does not leak into our domain.
// ======================================================================

builder.Services.AddHttpClient<IErpClient, ErpClient>(c =>
{
    c.BaseAddress = new Uri(builder.Configuration["Erp:BaseUrl"]!);
    c.Timeout = Timeout.InfiniteTimeSpan; // the timeout is controlled by the resilience pipeline
})
.AddStandardResilienceHandler(o =>         // Microsoft.Extensions.Http.Resilience (Polly v8)
{
    o.Retry.MaxRetryAttempts = 3;
    o.Retry.UseJitter = true;
    o.AttemptTimeout.Timeout = TimeSpan.FromSeconds(10);
    o.CircuitBreaker.SamplingDuration = TimeSpan.FromSeconds(30);
});