Refactor async void methods to async Task in PowerRenameViewModel

[Copilot]

Summary of the Pull Request

Converts InstallHeifExtension and InstallAvifExtension from async void to async Task methods, addressing exception handling and testability concerns raised in PR #44466.

PR Checklist

• [ ] Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• [ ] Tests: Added/updated and all pass
• [ ] Localization: All end-user-facing strings can be localized
• [ ] Dev docs: Added/updated
• [ ] New binaries: Added on the required places
  • [ ] JSON for signing for new binaries
  • [ ] WXS for installer for new binaries and localization folder
  • [ ] YML for CI pipeline for new test projects
  • [ ] YML for signed pipeline
• [ ] Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Async void methods prevent proper exception propagation and cannot be awaited in tests. Changed both extension installer methods to return Task and use AsyncCommand instead of RelayCommand.

Before:

public ICommand InstallHeifExtensionCommand => new RelayCommand(InstallHeifExtension);

private async void InstallHeifExtension()
{
    try
    {
        await Launcher.LaunchUriAsync(new Uri(HeifExtensionStoreUri));
    }
    catch (Exception ex)
    {
        Logger.LogError("Failed to open HEIF extension store page", ex);
    }
}

After:

public ICommand InstallHeifExtensionCommand => new AsyncCommand(InstallHeifExtensionAsync);

private async Task InstallHeifExtensionAsync()
{
    try
    {
        await Launcher.LaunchUriAsync(new Uri(HeifExtensionStoreUri));
    }
    catch (Exception ex)
    {
        Logger.LogError("Failed to open HEIF extension store page", ex);
    }
}

Pattern follows existing AsyncCommand usage in MouseWithoutBordersPage.xaml.cs.

Validation Steps Performed

• Code review: No issues
• Security scan: No issues
• Follows existing codebase patterns

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