feat: support conversations: keep in deleteUsers

[Acconut]

CLA

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• Code changes are tested

Description of the changes, What, Why and How?

The backend adds a new conversations: 'keep' option to POST /users/delete (GetStream/chat#18338). It lets user: 'hard' delete a user without deleting their conversation channels (channels with two or fewer members). An unset conversations keeps defaulting to hard for hard user deletes, so keep has to be passed explicitly.

deleteUsers validates conversations on the client and rejects anything other than soft and hard, so keep never reaches the API. This adds keep to the client-side check and to the DeleteUserOptions type and doc comment.

Do not merge before GetStream/chat#18338 is deployed. Until then the API rejects keep with a 400.

Changelog

• Support conversations: 'keep' in deleteUsers