[docs] Fix Subscribe syntax bug and adopt OnConnectionStringAvailable/OnInitializeResource helpers

[IEvangelist]

Fixes #315

Summary

Issue #315 asked us to refresh the eventing docs to use the new resource-level helper methods (OnInitializeResource, OnConnectionStringAvailable, OnResourceReady, etc.) and to sweep remaining Eventing.Subscribe<T> usages out of the docs where a helper now applies.

PR #821 already modernized the main eventing landing page and added the helper table, but:

1. It left a regression in the lower-level fallback example — the Eventing.Subscribe<AfterResourcesCreatedEvent>(...) call is missing its closing });, so the snippet as published on https://aspire.dev/app-host/eventing/ doesn't compile.
2. It didn't reach the other docs the issue called out:
   • architecture/resource-examples.mdx still uses Eventing.Subscribe<ConnectionStringAvailableEvent>(redis, ...).
   • extensibility/custom-resources.mdx uses Eventing.Subscribe<AfterResourcesCreatedEvent>(resource, ...), which is a compile error today — AfterResourcesCreatedEvent doesn't implement IDistributedApplicationResourceEvent, so the resource overload of Subscribe won't accept it.

This PR fixes the three issues above and leaves all Eventing.Subscribe<T> usages in place where no helper currently exists (e.g. AfterEndpointsAllocatedEvent, AfterResourcesCreatedEvent at the AppHost level) or where it's intentional historical context (release-notes pages, low-level pattern references).

Changes

• src/frontend/src/content/docs/app-host/eventing.mdx — adds the missing }); that closes the Eventing.Subscribe<AfterResourcesCreatedEvent>(...) lower-level fallback example introduced in [docs] AppHost eventing: document OnBeforeStart, OnBeforePublish, OnAfterPublish builder extension methods #821.
• src/frontend/src/content/docs/architecture/resource-examples.mdx — replaces the standalone builder.Eventing.Subscribe<ConnectionStringAvailableEvent>(redis, ...) block with OnConnectionStringAvailable(...) chained into the resource's fluent configuration. Renumbers the inline step comments (4 → 4 / 5 → 5.a–5.h) so the prose still tracks the walkthrough's structure.
• src/frontend/src/content/docs/extensibility/custom-resources.mdx — rewrites the "Using inline event subscriptions" example to use OnInitializeResource((resource, @event, ct) => ...), which actually compiles and is semantically correct for the section's intent (per-resource one-time initialization). Adds a sentence listing the resource-level helpers so readers can see the full set.

Evidence the bugs are live

Captured from https://aspire.dev on the day of this PR (the published HTML for the same code blocks):

https://aspire.dev/app-host/eventing/ — missing });:

builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
    async (@event, ct) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("AfterResourcesCreatedEvent");
        return Task.CompletedTask;

builder.Build().Run();

https://aspire.dev/architecture/resource-examples/ — uses old API:

builder.Eventing.Subscribe<ConnectionStringAvailableEvent>(redis, async (@event, ct) =>
{
    // ...
});

https://aspire.dev/extensibility/custom-resources/ — uses the resource overload with an event type that doesn't satisfy the generic constraint:

builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(resource, async (@event, ct) =>
{
    // Initialize mail server, create test mailboxes, etc.
});

How I verified

1. API surface check — read src/Aspire.Hosting/DistributedApplicationEventingExtensions.cs and src/Aspire.Hosting/Eventing/IDistributedApplicationEventing.cs in microsoft/aspire to enumerate the AppHost-level helpers (OnBeforeStart, OnBeforePublish, OnAfterPublish) and resource-level helpers (OnBeforeResourceStarted, OnResourceStopped, OnConnectionStringAvailable, OnInitializeResource, OnResourceEndpointsAllocated, OnResourceReady), and to confirm the generic constraints (T : IDistributedApplicationEvent vs T : IDistributedApplicationResourceEvent vs T : IResourceWithConnectionString).

2. Compile-check — scaffolded an empty AppHost with aspire new aspire-empty --language csharp and copied each of the modified code patterns into it (Subscribe<AfterResourcesCreatedEvent>(...) with the closing });, OnBeforeStart/OnBeforePublish/OnAfterPublish on the builder, OnConnectionStringAvailable((resource, @event, ct) => ...) on a RedisResource chain, OnInitializeResource((resource, @event, ct) => ...) on a resource builder). dotnet build succeeds with no errors or warnings.

3. Live regression confirmation — fetched the three pages on https://aspire.dev directly and observed the broken/outdated code blocks above.

Notes

• architecture/resource-hierarchies.mdx still uses Eventing.Subscribe<AfterEndpointsAllocatedEvent>(...) — left as-is, because no helper exists for AfterEndpointsAllocatedEvent.
• architecture/resource-api-patterns.mdx references Subscribe<T> deliberately as the underlying pattern — left as-is.
• whats-new/*.mdx release notes also reference Eventing.Subscribe<T> — left as-is; these are historical and intentionally describe the API of that release.
• The remaining Subscribe<ResourceStoppedEvent>(cache, ...) example near the bottom of app-host/eventing.mdx (line ~416) could be modernized to cache.OnResourceStopped(...). I left it alone to keep this PR scoped to the issue's reported defects; happy to fold it in if reviewers prefer.

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Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com