fix(api/endpoint): stop silently discarding non-map form bodies

stringify_values/1 had a catch-all that coerced any non-map payload to
%{}, silently dropping all form fields on the files: :form path. Narrow
the catch-all to nil only (the legitimate payload || %{} case); any other
non-map now raises FunctionClauseError instead of vanishing on the wire.

Add prepare_body/3 form-field tests covering the map, nil, and
non-map-raises cases.

lib/dexcord/api/endpoint.ex

@@ -251,7 +251,9 @@ defmodule Dexcord.Api.Endpoint do
   defp stringify_values(payload) when is_map(payload),
     do: Map.new(payload, fn {k, v} -> {k, to_string(v)} end)
 
-  defp stringify_values(_payload), do: %{}
+  # A nil payload is legitimate (payload || %{} semantics); any other non-map
+  # is a caller error and must raise rather than silently drop the form fields.
+  defp stringify_values(nil), do: %{}
 
   @doc false
   def encode_body(nil, _spec), do: nil

test/dexcord/endpoint_test.exs

@@ -136,6 +136,42 @@ defmodule Dexcord.Api.EndpointTest do
     end
   end
 
+  describe "prepare_body/3 with form-field files (sticker create)" do
+    defp form_spec do
+      %{body: true, files: :form, binary_wrap: nil}
+    end
+
+    test "stringifies every payload value onto bare form parts" do
+      file = %{filename: "s.png", data: <<1>>}
+
+      assert {:form_multipart, fields, out_file} =
+               Endpoint.prepare_body(form_spec(), [name: "sticker", tags: "cat"], files: [file])
+
+      assert fields == %{"name" => "sticker", "tags" => "cat"}
+      assert out_file.filename == "s.png"
+      refute Map.has_key?(out_file, :description)
+    end
+
+    test "a nil body is legitimate and yields empty form fields" do
+      file = %{filename: "s.png", data: <<1>>}
+
+      assert {:form_multipart, %{}, out_file} =
+               Endpoint.prepare_body(form_spec(), nil, files: [file])
+
+      assert out_file.filename == "s.png"
+    end
+
+    test "a non-map, non-nil payload raises rather than silently vanishing" do
+      file = %{filename: "s.png", data: <<1>>}
+      # A top-level list body encodes to a list, which must not be coerced to %{}.
+      assert_raise FunctionClauseError, fn ->
+        Endpoint.prepare_body(form_spec(), [%Dexcord.Test.Gadget{id: 1, name: "a"}],
+          files: [file]
+        )
+      end
+    end
+  end
+
   describe "decode_response/2 (AC1.6 + passthrough)" do
     test "decodes a single map to the returns struct" do
       assert {:ok, %Dexcord.User{id: 1}} =