fix #19481: stop audio on previewer navigation

[Ayush-Patel-56]

Purpose / Description

Audio currently continues playing in the Previewer even when navigating to other cards or switching views. This change ensures playback stops immediately when the user navigates away or toggles the card side.

Fixes

• Fixes Audio on the front side of a card continues to play on the back side, in the browser preview #19481

Approach

• Calling cardMediaPlayer.stop() in onNextButtonClick, onPreviousButtonClick, and toggleBackSideOnly.
• Updated PreviewerViewModelTest to verify stop() is called during these actions.

How Has This Been Tested?

Expanded PreviewerViewModelTest to cover these navigation scenarios and verified they pass.

Video Proof:
https://github.com/user-attachments/assets/44304669-c881-4533-b13f-1e08682210fb

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

It would seem sensible to me to call this before any calls to super.showQuestion or super.showAnswer, likely be overriding the method.

@BrayanDSO could I confirm this wouldn't cause an issue?

[Ayush-Patel-56]

Done!, I've refactored the audio stop logic into showQuestion/showAnswer overrides as suggested. Also added regression test.