api!: jsonrpc: remove `contacts` from `FullChat`. To migrate load contacts on demand via `get_contacts_by_ids` using `FullChat.contactIds`

[Simon-Laux]

TO DO:

• [x] check if existing tests pass on this pr
• [x] check if example bots use this (echo bot)
• [ ] desktop pr https://github.com/deltachat/deltachat-desktop/pull/5527

closes #6945

Why not deprecate it instead?

Because empty contact list is a more annoying-to-find bug in your app than failing to build or getting undefined at runtime.

Also you would not see the deprecated hints anyway because for that you need autogenerated types and those only exist for typescript currently.