Use Assert.Fail in WebView integration test

[Redth]

The Windows DevFlow integration suite passes, but the local run surfaced an xUnit analyzer warning in the WebView integration test helper. This change keeps the test behavior the same while using the analyzer-preferred assertion API.

Summary

• Replaces Assert.True(false, message) with Assert.Fail(message) in WebViewTests.
• Leaves the timeout failure message and fallback flow unchanged.

Testing

• DEVFLOW_TEST_PLATFORM=windows dotnet test src\DevFlow\Microsoft.Maui.DevFlow.Agent.IntegrationTests\Microsoft.Maui.DevFlow.Agent.IntegrationTests.csproj - 113/113 passed
• Re-ran the same Windows integration suite after the fix - 113/113 passed, build succeeded with no warnings

[Copilot]

Pull request overview

This PR updates a DevFlow Windows WebView integration test helper to use the analyzer-preferred xUnit assertion API while preserving the existing timeout/fallback behavior.

Changes:

• Replaces Assert.True(false, message) with Assert.Fail(message) when no WebView contexts are detected within the timeout.

[github-actions]

✅ Expert Code Review: 3 independent reviewers found no issues. Methodology: 3-model adversarial consensus.

Generated by Expert Code Review (auto) for issue #286 · ● 2.3M · ◷

[Redth]

Thanks for the review. No actionable issues were reported, so no changes are needed.