Fix for Unobserved NavigationException in Blazor SSR

[ilonatommy]

Prevent rethrowing NavigationException in finalizer thread

Description

When a NavigationException is thrown from an async component lifecycle method during Blazor server-side rendering, it can become an unobserved task exception if not properly handled. This happens specifically in circular redirection scenarios, where the exception is thrown and then the task is garbage collected before being properly observed.

The issue manifests as an unhandled exception in the finalizer thread with a stack trace like:

Unhandled caught: System.AggregateException: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread.
 ---> Microsoft.AspNetCore.Components.NavigationException: Exception of type 'Microsoft.AspNetCore.Components.NavigationException' was thrown.
   at Microsoft.AspNetCore.Components.Endpoints.HttpNavigationManager.NavigateToCore(String uri, NavigationOptions options)
   ...
   at Microsoft.AspNetCore.Components.Endpoints.EndpointHtmlRenderer.<WaitForNonStreamingPendingTasks>g__Execute|43_0()
   at Microsoft.AspNetCore.Components.Endpoints.EndpointHtmlRenderer.WaitForResultReady(Boolean waitForQuiescence, PrerenderedComponentHtmlContent result)

Root Cause

The issue occurs in the WaitForNonStreamingPendingTasks method in EndpointHtmlRenderer, where we await a collection of tasks without properly handling NavigationExceptions that might be thrown from them. When the task is garbage collected before completing, the unobserved exception is rethrown from the finalizer thread.

Fix

We've added proper exception handling in WaitForNonStreamingPendingTasks to specifically catch, observe NavigationExceptions and handle navigation when possible.

Validation

We added NavigationException_InAsyncContext_DoesNotBecomeUnobservedTaskException test that verifies no unobserved exceptions occur when a component causes circular redirections, even with forced garbage collection. We set the AppContext switch to make sure navigation uses the exception-driven flow.

Fixes #62167

[ilonatommy]

/ba-g Unrelated build failure

[javiercn]

Is this specific to the EndpointHtmlRenderer? Or could it happen in one of the interactive modes

[ilonatommy]

Interactive is not throwing NavigationException to redirect. I don't know how it could happen there.