fix: 修复视频画面拉伸变形问题

移除视频预览层的aspectRatio约束，并将本地解码画面渲染模式从resizeAspect改为resizeAspectFill，确保视频内容正确填充显示区域。

webrtc_controller_ios/web_rtc_controller_ios/Views/SelfCodecDisplayView.swift

@@ -16,13 +16,13 @@ final class SampleBufferDisplayView: UIView {
 
     override init(frame: CGRect) {
         super.init(frame: frame)
-        sampleBufferLayer.videoGravity = .resizeAspect
+        sampleBufferLayer.videoGravity = .resizeAspectFill
         backgroundColor = .black
     }
 
     required init?(coder: NSCoder) {
         super.init(coder: coder)
-        sampleBufferLayer.videoGravity = .resizeAspect
+        sampleBufferLayer.videoGravity = .resizeAspectFill
         backgroundColor = .black
     }
 }