Fix PyAV frame rate handling in video writer

[ModifierLTD]

Summary

• pass a rational frame rate to PyAV when creating H.264 output streams
• avoid the AttributeError raised by newer PyAV versions when rate is passed as a formatted string

Verification

• ran python inference.py --variant mobilenetv3 --checkpoint rvm_mobilenetv3.pth --device cuda --input-source input.mp4 --output-type video --output-composition composition.mp4 --output-alpha alpha.mp4 --output-foreground foreground.mp4
• ran python inference.py --variant resnet50 --checkpoint rvm_resnet50.pth --device cuda --input-source input.mp4 --output-type video --output-composition composition.mp4 --output-alpha alpha.mp4 --output-foreground foreground.mp4
• confirmed output metadata matches the input video and sampled frame statistics show non-degenerate alpha, foreground, and composition outputs

Artifacts

• Conversation: https://app.warp.dev/conversation/798a5372-c0c4-46c7-a2eb-deba11e3a235

Co-Authored-By: Oz oz-agent@warp.dev