Surface app startup exceptions after the host is disposed in WebApplicationFactory

[Demivan]

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Rethrow the app's startup exception instead of ObjectDisposedException

Description

When the app under test fails in IHost.StartAsync, app.Run() disposes the host before the entry point exits. If that happens before DeferredHost.StartAsync resolves IHostApplicationLifetime, WebApplicationFactory throws ObjectDisposedException and the real startup exception is lost.

DeferredHost.StartAsync now skips the ApplicationStarted registration when the host is already disposed and keeps awaiting the start TCS. The entry point is exiting at that point, and HostFactoryResolver reports its completion in a finally, so EntryPointCompleted always completes the TCS with the app's exception or result. The success and cancellation paths are unchanged.

The regression test uses SimpleWebSiteWithWebApplicationBuilder with a failing ValidateOnStart. Its factory holds CreateHost until the app's service provider has been disposed, which makes the ordering deterministic. Without the fix it failed 10/10 runs with the reported ObjectDisposedException stack. With the fix it passed 10/10, and Mvc.FunctionalTests passes in full (1669 passed, 3 skipped).

Fixes #69684

🤖 Generated with Claude Code

[dotnet-policy-service]

Thanks for your PR, @Demivan. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

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