fix: use IO.iodata_to_binary before pattern-matching format output

The pattern-matching on `Code.format_string!/2` output directly works on
Elixir 1.19 where dialyzer infers binary() | nonempty_improper_list(),
but fails on Elixir 1.18 where the return type is [binary()] and the
"" pattern cannot match. Normalize to binary first so the check is
version-agnostic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: fahchen

lib/dprint_markdown_formatter.ex

@@ -255,9 +255,14 @@ defmodule DprintMarkdownFormatter do
       # Remove dprint options to pass only valid Elixir formatter options
       elixir_opts = Keyword.drop(opts, @dprint_opts)
 
-      case Code.format_string!(formatted_content, elixir_opts) do
+      formatted =
+        formatted_content
+        |> Code.format_string!(elixir_opts)
+        |> IO.iodata_to_binary()
+
+      case formatted do
         "" -> {:ok, ""}
-        formatted -> {:ok, IO.iodata_to_binary([formatted, ?\n])}
+        _ -> {:ok, formatted <> "\n"}
       end
     end
   end