Debugging a .NET pod in CrashLoopBackOff on Kubernetes (C# ValidateOnStart)

// ======================================================================
// Q: A pod is in CrashLoopBackOff. What do you do?
//
// - Exception at startup (missing config or secret): logs --previous shows
//   InvalidOperationException or OptionsValidationException.
// - OOMKilled: describe shows Last State: Terminated, Reason: OOMKilled.
// - Liveness probe too strict: Events show Liveness probe failed, with restarts
//   and no exception in the logs.
// - Wrong port: the probe fails; the app listens on 8080 but the manifest says
//   80.
// - ImagePullBackOff: wrong tag or missing access to the registry (ACR).
// ValidateOnStart() on Options makes a bad setting show up immediately at
// startup with a clear message, instead of later on the first request.
// ======================================================================

// Fail fast on invalid configuration
builder.Services.AddOptions<ErpOptions>()
    .BindConfiguration("Erp")
    .ValidateDataAnnotations()
    .ValidateOnStart();          // the pod fails immediately, with a clear message in the logs