Using `InteractionSerivce` in `BeforeStart` deadlocks

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

If you try to use the interaction service in `BeforeStart`, it will deadlock the app host, even if you guard the prompt with `interactionService.IsAvailable`.

### Expected Behavior

`interactionService.IsAvailable` should return false if the dashboard isn't running (such as in `BeforeStart`) (or has failed for any reason).

### Steps To Reproduce

```cs
builder.Eventing.Subscribe(async (evt, ct) =>
{
var interactionService = evt.Services.GetRequiredService();

if (!interactionService.IsAvailable)
{
return;
}

await interactionService.PromptMessageBoxAsync(
title: "👋🌍",
message: "Hello World",
options: new MessageBoxInteractionOptions
{
EnableMessageMarkdown = true,
Intent = MessageIntent.Error,
});
});
```

### Exceptions (if any)

_No response_

### .NET Version info

_No response_

### Anything else?

_No response_

Start with the BeforeStart event subscription and the IInteractionService implementation used by the dashboard; the payload does not name specific files or tests. Reproduce the example, then trace IsAvailable and the prompt call. Done means IsAvailable is false when the dashboard is unavailable, including during BeforeStart, and the app host does not deadlock.

Written by the indexing model from the issue text.