Prompt completion popup state for @-mention or /slash completion.
candidates are either {name, description} tuples or bare name strings.
Summary
Types
@type t() :: %Minga.RenderModel.UI.AgentChat.PromptCompletion{ anchor_col: non_neg_integer() | nil, anchor_line: non_neg_integer() | nil, candidates: [candidate()], selected: non_neg_integer(), type: :mention | :slash }