Add test for Blazor web app Error page rendering with [PersistentState]

[Copilot]

Description

Adds a new test BlazorWebTemplate_ErrorPage_RendersCorrectly to BlazorWebTemplateTest.cs to verify the Error page in the Blazor web app template renders correctly.

This test was requested as a review comment on PR #66245, which fixed Error.razor by making the RequestId property public. The [PersistentState] attribute requires a public getter — when private, PersistentValueProviderComponentSubscription.PropertyGetterFactory would throw:

InvalidOperationException: A public property 'RequestId' on component type '...' with a public getter wasn't found.

The test:

1. Creates a Blazor web app with Server interactivity (where [PersistentState] is exercised across the prerender → circuit hydration boundary)
2. Navigates to /Error via Playwright
3. Asserts the page renders with h1.text-danger and h2.text-danger content
4. Asserts the Request ID is displayed — this directly validates that [PersistentState] on the public RequestId property is functioning; a private property would throw before the page could render

Changes

• src/ProjectTemplates/test/Templates.Blazor.Tests/BlazorWebTemplateTest.cs: New BlazorWebTemplate_ErrorPage_RendersCorrectly test

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[Copilot]

Pull request overview

Adds an end-to-end Playwright test to the Blazor web app template test suite to validate that the /Error page renders successfully under Server interactivity, covering the [PersistentState]/RequestId regression scenario from #66245.

Changes:

• Add a new BlazorWebTemplate_ErrorPage_RendersCorrectly Playwright test that navigates to /Error and asserts expected headings and the presence of “Request ID”.
• Add the necessary Microsoft.Playwright using for WaitUntilState usage in the new test.

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