add Printify client test coverage with Req.Test stubs

Same pattern as the Printful work: wire up base_options/0 so tests can
inject a Req.Test plug, fix unreachable 204 clause in delete, add
HTTP-level client tests and provider integration tests.

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

lib/berrypod/clients/printify.ex

@@ -26,7 +26,7 @@ defmodule Berrypod.Clients.Printify 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, body}
 
@@ -44,7 +44,10 @@ defmodule Berrypod.Clients.Printify 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, body}
 
@@ -62,7 +65,10 @@ defmodule Berrypod.Clients.Printify do
   def put(path, body, _opts \\ []) do
     url = @base_url <> path
 
-    case Req.put(url, json: body, headers: auth_headers(), receive_timeout: 60_000) do
+    case Req.put(
+           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, body}
 
@@ -80,13 +86,13 @@ defmodule Berrypod.Clients.Printify do
   def delete(path, _opts \\ []) do
     url = @base_url <> path
 
-    case Req.delete(url, headers: auth_headers(), receive_timeout: 30_000) do
+    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, body}
 
-      {:ok, %Req.Response{status: status}} when status == 204 ->
-        {:ok, nil}
-
       {:ok, %Req.Response{status: status, body: body}} ->
         {:error, {status, body}}
 
@@ -267,4 +273,8 @@ defmodule Berrypod.Clients.Printify do
       {"Content-Type", "application/json"}
     ]
   end
+
+  defp base_options do
+    Application.get_env(:berrypod, __MODULE__, [])[:req_options] || []
+  end
 end