Fix process_video and add test case

[cdeil]

This PR fixes the process_video function.

There used to be an if-else but somewhere the if fell off the truck resulting in a for-else that doesn't make sense.

This PR also fixes the following case of passing a max_frames that is larger than the frames in the video. Before this PR is would give Exception: Requested frames are outbound and with it it will simply process the whole video, i.e. treat "max" frames as a max but OK if there are less frames (I think what most users expect/want).

import supervision as sv
from supervision.assets import VideoAssets, download_assets

download_assets(VideoAssets.PEOPLE_WALKING)

video_info = sv.VideoInfo.from_video_path("people-walking.mp4")

sv.process_video(
    source_path="people-walking.mp4",
    target_path="people-talking.mp4",
    callback=lambda frame, _: frame,
    max_frames=1000,
)

I also added a test case (the agent wrote it). Not sure if you want such tests or if/how you want to treat temp dirs and example asset video files in this repo. Let me know whatever you prefer and I can remove that or adjust.