[camera_windows] Fixes initializing video preview with latest webcam driver

[dasyad00]

Explanation post: flutter/flutter#140014 (comment)

Resolves #140014.

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request addresses an issue with video preview initialization on Windows for certain webcam drivers. The fix involves explicitly querying for video, photo, and audio stream indices instead of relying on predefined constants. A key change is the addition of a call to SetCurrentDeviceMediaType after determining the best media type for preview. These changes are correctly propagated through the photo, preview, and record handlers, and the corresponding tests have been updated. I've identified one potential issue where the search for an audio stream is unconditional, which could cause problems in video-only capture scenarios.