Feature request: `video_cover` and `video_start_timestamp` support in `Message.copy`

[JeoriTr]

Feature request: Support video_cover and video_start_timestamp in Message.copy

When using Message.copy() for video messages, it is currently not possible to:

• Set or override a video thumbnail/cover when copying a message.
• Set a custom start timestamp from which the copied video should start playing.

Proposed enhancement

Extend the Message.copy()/Client.copy_message() implementation to support the following parameters when the source message contains a video:

• video_cover: Optional thumbnail/cover for the copied video (e.g. file path / bytes / suitable input type consistent with existing media upload parameters in pyrofork).
• video_start_timestamp: Optional integer timestamp (in seconds) indicating from which point the copied video should start playing.

These options would make it possible to:

• Change or add a thumbnail when reusing existing video messages.
• Create "preview" copies of long videos that start playback from a specific point.

Possible API surface

On the bound method side:

await message.copy(
    chat_id,
    video_cover="thumb.jpg",  # or other supported input type
    video_start_timestamp=120,
)

On the client method side:

await client.copy_message(
    chat_id=chat_id,
    from_chat_id=message.chat.id,
    message_id=message.id,
    video_cover=...,  # new
    video_start_timestamp=...,  # new
)

If these parameters cannot be supported for all media types, limiting them to regular video messages would still be very useful.

Thank you for considering this feature.