add Printful client test coverage with Req.Test stubs

Wire up Req.Test plug for the Printful HTTP client so tests can stub
responses. Adds HTTP-level tests for the client, provider integration
tests, and mockup enricher tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/clients/printful.ex

@@ -41,7 +41,7 @@ defmodule Berrypod.Clients.Printful do
   def get(path, _opts \\ []) do
     url = @base_url <> path
 
-    case Req.get(url, headers: auth_headers(), receive_timeout: 30_000) do
+    case Req.get(url, [headers: auth_headers(), receive_timeout: 30_000] ++ base_options()) do
       {:ok, %Req.Response{status: status, body: body}} when status in 200..299 ->
         {:ok, unwrap_response(path, body)}
 
@@ -59,7 +59,10 @@ defmodule Berrypod.Clients.Printful do
   def post(path, body, _opts \\ []) do
     url = @base_url <> path
 
-    case Req.post(url, json: body, headers: auth_headers(), receive_timeout: 60_000) do
+    case Req.post(
+           url,
+           [json: body, headers: auth_headers(), receive_timeout: 60_000] ++ base_options()
+         ) do
       {:ok, %Req.Response{status: status, body: body}} when status in 200..299 ->
         {:ok, unwrap_response(path, body)}
 
@@ -77,13 +80,13 @@ defmodule Berrypod.Clients.Printful do
   def delete(path, _opts \\ []) do
     url = @base_url <> path
 
-    case Req.delete(url, headers: auth_headers(), receive_timeout: 30_000) do
-      {:ok, %Req.Response{status: status, body: body}} when status in 200..299 ->
-        {:ok, unwrap_response(path, body)}
-
+    case Req.delete(url, [headers: auth_headers(), receive_timeout: 30_000] ++ base_options()) do
       {:ok, %Req.Response{status: 204}} ->
         {:ok, nil}
 
+      {:ok, %Req.Response{status: status, body: body}} when status in 200..299 ->
+        {:ok, unwrap_response(path, body)}
+
       {:ok, %Req.Response{status: status, body: body}} ->
         {:error, {status, body}}
 
@@ -362,4 +365,8 @@ defmodule Berrypod.Clients.Printful do
       id -> [{"X-PF-Store-Id", to_string(id)} | headers]
     end
   end
+
+  defp base_options do
+    Application.get_env(:berrypod, __MODULE__, [])[:req_options] || []
+  end
 end