Fix channel/chat IDs being misinterpreted as user IDs

[embogomolov]

What's broken

Telethon uses unmarked IDs internally — positive integers for everything. When the server returns these raw IDs to callers (say, from list_chats), and the caller passes them back into get_entity(int), Telethon sees a positive number and assumes it's a user. Channels and group chats get a ValueError: Could not find the input entity for PeerUser.

Any "list chats → pick one → do something" workflow breaks intermittently, especially after long sessions when entity cache expires.

The fix

get_marked_id() helper that converts entity IDs to Telethon's marked format before returning:

• Channels: -100{id} (PeerChannel)
• Chats: -{id} (PeerChat)
• Users: unchanged

Applied in all places where entity IDs go out to callers: list_chats, get_chats, get_chat, format_entity, get_direct_chat_by_contact, get_contact_chats, create_group, get_bot_info, get_folder.

How to test

• Call list_chats, grab a channel ID, pass it to search_messages or get_history — should work without errors
• Channel IDs now look like -1001676885811 instead of 1676885811

[embogomolov]

Anyone?

[chigwell]

Hey @embogomolov, thank you for the contribution! Could I please ask you to fix the black formatting so the CI tests will pass? Thank you!