Reorder popup/view model instantiation

[bijington]

Description of Change

This PR changes the order of instantiation to avoid unnecessary instantiations of the popups view model.

Linked Issues

• Fixes [BUG] PopupV2 PopupService.ShowPopupAsync<T> view model assigned to the popup is created twice. #3065

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

[Copilot]

Pull request overview

This PR adjusts how PopupService resolves popup content so that when a popup is shown via its registered ViewModel type, the ViewModel is not instantiated redundantly (fixing the double-creation behavior reported in #3065).

Changes:

• Updated ShowPopup*/ShowPopupAsync* overloads to resolve popup content via GetPopupContent<T>() instead of resolving T up-front.
• Refactored GetPopupContent to resolve the mapped popup View first (when available) and only resolve T directly when needed.

[bijington]

@Copilot please address the comments

[TheCodeTraveler]

Thanks Shaun!

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

This call uses a named argument cancellationToken, but the ShowPopupAsync<T>(INavigation, ...) overload’s parameter is named token. As written this won’t compile; use the correct parameter name or pass the token positionally.

Suggested change

	await popupService.ShowPopupAsync<SingleConstructionViewModel>(page.Navigation, cancellationToken: TestContext.Current.CancellationToken);
	await popupService.ShowPopupAsync<SingleConstructionViewModel>(page.Navigation, token: TestContext.Current.CancellationToken);