Fix AV1 transcode validation: move -t option to output side of ffmpeg command

[Copilot]

AV1 files fail validation with "No sequence header" and "Corrupt frame detected" errors because the decode test limits input duration before the decoder can initialize.

Changes

• Move -t 2 from input to output position in the ffmpeg decode test command within validate_video_file()

When -t precedes -i, it limits bytes read from input. AV1's sequence header may not be reached in time. As an output option, ffmpeg reads enough to initialize the decoder properly.

# Before: -t as input option (breaks AV1)
decode_cmd = ['ffmpeg', '-v', 'error', '-t', '2', '-i', path, '-f', 'null', '-']

# After: -t as output option (allows decoder init)
decode_cmd = ['ffmpeg', '-v', 'error', '-i', path, '-t', '2', '-f', 'null', '-']

Original prompt

I am getting this errors when trying to transcode some files that were originally encoded in AV1. Any ideas or suggestions on a fix?

[libaom-av1 @ 0x55cdb09802c0] Failed to decode frame: Corrupt frame detected
[libaom-av1 @ 0x55cdb09802c0] Additional information: No sequence header
[vist#0:0/av1 @ 0x55cdb0de8040] Error submitting packet to decoder: Invalid data found when processing input

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