[lukaszsamson]

Completion crashed when a third party plugin returned a function/macro suggestion without the optional default_args key:

** (KeyError) key :default_args not found in:
    %{args: "value", arity: 1, name: "select", type: :function,
      origin: "Ash.Resource.Change.Builtins", args_list: ["value"], def_arity: 1, ...}
  completion.ex:596: anonymous fn/4 in Completion.maybe_reject_derived_functions/3
  completion.ex:280: Completion.completion/4

The suggestion has the old ElixirSense plugin shape (def_arity, args_list, origin, no default_args), supplied by a plugin loaded through ElixirSense.Plugin / ElixirLS.LanguageServer.Plugin. It is not emitted by Spark.ElixirSense.Plugin (spark 2.7.8 emits no function suggestions of this shape), so the exact emitting package is unconfirmed.

maybe_reject_derived_functions/3 now treats a missing default_args as 0. The other from_completion_item clauses pattern match on their keys and fall through to the catch-all clause, so they do not raise.

Adds a test with a fake plugin that emits this shape.