Stabilize Virtualize cancellation test case

[Yuvan111]

Stabilize Virtualize cancellation test case

Description

Stabilizes ScrollToIndexAsync_CancellationCancelsProviderRequest by removing the timeout-prone TaskCompletionSource rendezvous.

The provider now captures its cancellation token and triggers cancellation directly. This guarantees cancellation occurs only after the provider request has started, while still verifying that cancellation propagates to the in-flight provider request.

Fixes #68852

Testing

• Built Microsoft.AspNetCore.Components.Web.Tests
• Ran the targeted test successfully
• Ran the targeted test 100 times in a loop with zero failures
• Verified no diagnostics or whitespace errors

The original intermittent failure was not reproduced locally. Validation is based on structurally eliminating the reported timeout-prone synchronization boundary, with repeated execution as supporting evidence.

[dotnet-policy-service]

Thanks for your PR, @Yuvan111. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

🟢 Approval recommended

The change is isolated to test code, aligns with the stated stabilization goal, and preserves the core verification that cancellation propagates to the in-flight provider request.

Pull request overview

This pull request stabilizes the quarantined VirtualizeTest.ScrollToIndexAsync_CancellationCancelsProviderRequest test by removing a timeout-prone TaskCompletionSource “request started” rendezvous and instead initiating cancellation directly from inside the items provider once the provider request has definitively begun.

Changes:

• Replaced the TaskCompletionSource start/cancel coordination with a provider-captured cancellation token and in-provider CancellationTokenSource.Cancel() trigger.
• Simplified the assertion to verify the provider request’s cancellation token is observed as canceled.

File summaries

File	Description
src/Components/Web/test/Virtualization/VirtualizeTest.cs	Updates the cancellation propagation test to avoid timeout-prone synchronization and validate cancellation via the provider’s captured token.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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