Fix build error: correct StreamingRequestDto cast

The previous commit referenced state.Request.SegmentContainer which doesn't
exist. BaseEncodingJobOptions doesn't have a SegmentContainer property - only
StreamingRequestDto does.

Fixed by:
- Cast BaseRequest to StreamingRequestDto before accessing SegmentContainer
- Add using directive for MediaBrowser.Controller.Streaming namespace
- Use null-conditional operator to safely check SegmentContainer

MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs

@@ -20,6 +20,7 @@ using Jellyfin.Extensions;
 using MediaBrowser.Common.Configuration;
 using MediaBrowser.Controller.Extensions;
 using MediaBrowser.Controller.IO;
+using MediaBrowser.Controller.Streaming;
 using MediaBrowser.Model.Configuration;
 using MediaBrowser.Model.Dlna;
 using MediaBrowser.Model.Dto;
@@ -7484,7 +7485,8 @@ namespace MediaBrowser.Controller.MediaEncoding
             {
                 // Use fragmented MP4 for adaptive streaming (HLS/DASH with segments)
                 // Use faststart for progressive downloads (better seeking and metadata)
-                if (!string.IsNullOrEmpty(state.Request.SegmentContainer))
+                var streamingRequest = state.BaseRequest as StreamingRequestDto;
+                if (streamingRequest?.SegmentContainer is not null)
                 {
                     // Fragmented MP4 for HLS/DASH
                     format = " -f mp4 -movflags frag_keyframe+empty_moov+delay_moov";