Add autocomplete callback support

[Sim-hu]

Summary

Add an optional autocomplete/1 callback to the Nosedrum.ApplicationCommand behaviour, enabling command modules to handle autocomplete interactions separately from regular command invocations.

Motivation

Discord sends autocomplete interactions (type 4) when a user is typing in an option with autocomplete: true. Currently, nosedrum routes these to the same command/1 callback as regular command invocations, forcing command modules to manually inspect the interaction type and branch accordingly.

This is a pain point for anyone using autocomplete — the command logic and autocomplete logic serve different purposes and should be separate callbacks.

Changes

• lib/nosedrum/application_command.ex — add autocomplete/1 as an optional callback with documentation and example
• lib/nosedrum/storage/dispatcher.ex — split handle_interaction to detect autocomplete interactions (type 4) and route to autocomplete/1 if implemented, falling back to command/1 for backwards compatibility

Usage

defmodule MyApp.Commands.Search do
  @behaviour Nosedrum.ApplicationCommand

  @impl true
  def type, do: :slash

  @impl true
  def description, do: "Search for something"

  @impl true
  def options do
    [%{type: :string, name: "query", description: "Search query", required: true, autocomplete: true}]
  end

  @impl true
  def autocomplete(interaction) do
    focused = Enum.find(interaction.data.options, & &1.focused)
    results = MyApp.search(focused.value)
    [type: :application_command_autocomplete_result, choices: Enum.map(results, &%{name: &1, value: &1})]
  end

  @impl true
  def command(interaction) do
    [%{name: "query", value: query}] = interaction.data.options
    [content: "Results for: #{query}"]
  end
end

Backwards Compatibility

Fully backwards compatible — autocomplete/1 is an optional callback. If not implemented, autocomplete interactions fall back to command/1 as before.

Testing

• mix compile --warnings-as-errors passes
• mix test --no-start — 79 tests, 0 failures