Fix empty render of the ErrorBoundary

[dariatiurina]

Fix empty render of the ErrorBoundary

Description

This PR fixes a bug where ErrorBoundary renders empty content instead of ErrorContent when multiple child components throw errors in quick succession (e.g., errors thrown inside a foreach loop like in the test example).

How it works normally (single error):

The Renderer queues an empty render to clear the subtree, then calls HandleException, which queues the ErrorContent render. This works correctly.

The bug (multiple errors):

When a second error is thrown:

1. Renderer queues another empty render - this sets _hasPendingQueuedRender = true
2. Renderer calls HandleException to queue the ErrorContent render
3. StateHasChanged() returns early because _hasPendingQueuedRender is still true from step 1
4. The ErrorContent render is never queued

The queue ends up with: [empty, ErrorContent, empty] instead of [empty, ErrorContent, empty, ErrorContent]. After the first ErrorContent renders, only the empty render remains in the queue, clearing the error UI.

Solution:

After queuing the empty render but before calling HandleException, manually reset _hasPendingQueuedRender to false using the new internal HasPendingQueuedRender property. This ensures StateHasChanged() successfully queues the ErrorContent render for each error.

Fixes #56950

[oroztocil]

Is there a way how we could implemement this fix for ErrorBoundary components without reducing encapsulation for every component? E.g. by overriding StateHasChanged for ErrorBoundary.