Update eventing docs for new AppHost helper methods

[Copilot]

Aspire 13.2 introduces convenience methods for subscribing to AppHost lifecycle events (OnBeforeStart, OnAfterResourcesCreated, OnBeforePublish, OnAfterPublish) as simpler alternatives to builder.Eventing.Subscribe<T>.

Changes

• app-host/eventing.mdx: Updated code examples and migration guide to use new helper methods for AppHost-level events. Removed deprecated global event subscription patterns and simplified examples to focus on the recommended helper methods.
• architecture/resource-hierarchies.mdx: Simplified endpoint access pattern to use await endpoint.GetValueAsync() instead of event subscription for accessing allocated endpoint values.

Pattern

Before:

builder.Eventing.Subscribe<BeforeStartEvent>((@event, ct) => 
{
    // Handle event
    return Task.CompletedTask;
});

After:

builder.OnBeforeStart((@event, ct) => 
{
    // Handle event
    return Task.CompletedTask;
});

For accessing allocated endpoints:

Before:

builder.Eventing.Subscribe<ResourceEndpointsAllocatedEvent>((@event, ct) =>
{
    Console.WriteLine($"Url: {endpoint.Url}");
    return Task.CompletedTask;
});

After:

var allocatedEndpoint = await endpoint.GetValueAsync();
Console.WriteLine($"Url: {allocatedEndpoint.Url}");

Resource-specific event subscriptions (e.g., builder.Eventing.Subscribe<ConnectionStringAvailableEvent>(resource, handler)) remain unchanged as they have no AppHost-level equivalents.

Original prompt

Please work on issue #315

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[IEvangelist]

Tagging @afscrome

[afscrome]

A few comments. Pedantically, the ResourceEndpointsAllocatedEvent issues are not directly related to the linked PR so could be fixed in seperate PRs (potentially targeting main rather than 9.2).

[afscrome]

Given that ResourceEndpointsAllocatedEvent is deprecated, it's probably not the best first example to give of subscribing to events.

[afscrome]

For events that don't have a helper method, such as ResourceEndpointsAllocatedEvent, you can use the Eventing.Subscribe<T> API.

The only reason ResourceEndpointsAllocatedEvent doesn't have a helper is because the event is deprecated.

[afscrome]

Given that ResourceEndpointsAllocatedEvent is itself deprecated, this should probably recommend subscribing to the resource specific endpoint allocated event.

[afscrome]

Given that ResourceEndpointsAllocatedEvent is itself deprecated, this should probably recommend subscribing to the resource level events instead, noting that is not a drop in replacement like the other events.

[afscrome]

The ResourceEndpointsAllocatedEvent is a resource level event rather than an app wide event, so the code sample and surrounding docs probably needs more updates than this.

Probably don't even need to use the event as you can await endpoint.GetValueAsync(), rather than having to evaluate the endpoint in ResourceEndpointsAllocatedEvent.

[IEvangelist]

Hey @copilot, please update this based on the comments from
afscrome who is familiar with the source code.

[Copilot]

Hey @copilot, please update this based on the comments from
afscrome who is familiar with the source code.

Updated in commit 6c81c49. I've addressed all of @afscrome's feedback:

1. Removed ResourceEndpointsAllocatedEvent from the first example in eventing.mdx
2. Simplified the AppHost events section to focus on the new helper methods
3. Updated the migration guide to recommend resource-level OnResourceEndpointsAllocated(...)
4. Simplified the resource-hierarchies.mdx example to use await endpoint.GetValueAsync() instead of event subscription

The documentation now shows the recommended patterns without the deprecated/less-preferred approaches.

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