feat: runtime event subscription management (ARM control-plane facade)

[we4sz]

Summary

Adds an optional ARM control-plane (management) API so the real Azure.ResourceManager.EventGrid client can create, get, list and delete topic-scoped WebHook event subscriptions at runtime against the simulator — simply by repointing the client at a new managementPort. Today subscribers can only be configured at boot via appsettings.json; this closes the gap for apps that register/unregister subscriptions programmatically (e.g. self-registering webhooks) and want local/CI parity without touching their business code.

Disabled by default — if managementPort is not set, nothing changes.

Why

Azure Event Grid has two separate APIs: the data plane (the topic endpoint that receives events — already simulated) and the control plane (ARM, management.azure.com, where subscriptions are managed at runtime). The simulator only had the data plane, so code that manages subscriptions via Azure.ResourceManager.EventGrid had to be stubbed out locally. This adds a faithful-enough control-plane facade so that client works unmodified.

How

• New managementPort setting hosts a dedicated HTTPS listener, isolated from the per-topic data-plane pipeline via an EventGridMiddleware bypass — mirroring how Azure splits management.azure.com from the topic endpoint.
• EventSubscriptionsManagementController speaks the exact HTTP the SDK emits (the TopicEventSubscriptions operation group, api-version 2025-02-15), with synchronous, LRO-compatible responses so WaitUntil.Completed finalizes immediately without polling.
• ARM ⇄ HttpSubscriberSettings mapper (WebHook destinations), plus an in-memory, thread-safe copy-on-write subscriber registry seeded from config at boot — so the existing delivery path picks up runtime changes with no change to delivery code.
• Validation handshake on create: runtime-created WebHook subscriptions run the normal SubscriptionValidationEvent handshake before they receive events. The outbound handshake was extracted from ValidateAllSubscriptionsCommandHandler into a reusable SubscriptionValidationSender, now shared by the boot-time validator and the management API.
• Follows existing conventions: custom mediator (command + handler), thin controllers, [JsonPropertyName] settings, CSharpier, Constants.SupportedManagementApiVersion.

Scope / fidelity

Intentionally pragmatic — covers exactly what real callers use:

• Topic scope, WebHook destinations only. Domains/system/namespace/partner topics, non-WebHook destinations via ARM, true async provisioning operations, and AAD token validation are out of scope (the bearer token is accepted but not validated — this is a local dev simulator).
• Runtime subscriptions are in-memory and reset on restart (seeded from config).
• The WebHook endpointUrl is echoed back on GET (Azure treats it write-only) so the read-modify-write update pattern works against the simulator.

Config

{
  "managementPort": 60100,
  "topics": [ { "name": "MyTopic", "port": 60101, "key": "TheLocal+DevelopmentKey=" } ]
}

var options = new ArmClientOptions
{
    Environment = new ArmEnvironment(new Uri("https://localhost:60100/"), "https://management.azure.com"),
};
var arm = new ArmClient(credential, subscriptionId, options); // any token; not validated
await arm.GetEventGridTopicResource(new ResourceIdentifier(topicResourceId))
    .GetTopicEventSubscriptions()
    .CreateOrUpdateAsync(WaitUntil.Completed, "my-sub", data);

Testing

Driven by the real Azure.ResourceManager.EventGrid client against an actual running simulator instance:

• AzureResourceManagerEventGridTest — full create / get / list / delete round-trip (proves wire compatibility + LRO finalization).
• RuntimeSubscriptionDeliveryTest — end-to-end: create a subscription, publish an event, assert it is delivered to the runtime-created webhook, delete, assert delivery stops.
• Unit tests for the ARM↔subscriber mapper, the registry (including concurrent mutate-while-enumerate), and managementPort validation.

Full suite: 924 passing, 0 failing. README updated with a "Runtime subscription management" section.

Checklist

• Tests pass (dotnet test)
• Build succeeds (dotnet build)
• Commit messages use conventional format (feat:, fix:, chore:, etc.)
• Documentation updated (README)

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