Resend: no attachments with batch sending

Sometime earlier today, Resend started issuing API errors when sending attachments
with batch send. (Apparently the feature has never been supported, and attachments
were just ignored in the batch endpoint.)

Add a note to the docs, and update the Resend integration test.

docs/esps/resend.rst

@@ -189,6 +189,12 @@ anyway---see :ref:`unsupported-features`.
   .. versionchanged:: 12.0
       Resend now supports :attr:`~anymail.message.AnymailMessage.send_at`.
 
+**No attachments with batch sending**
+  Resend does not currently support attachments when using
+  :ref:`batch sending <resend-templates>`. Trying to send an attachment
+  while using :attr:`~anymail.message.AnymailMessage.merge_metadata` may
+  result in a Resend API error.
+
 **No envelope sender**
   Resend does not support specifying the
   :attr:`~anymail.message.AnymailMessage.envelope_sender`.

tests/test_resend_integration.py

@@ -115,7 +115,8 @@ def test_batch_send(self):
             },
         )
         message.attach_alternative("<p>HTML content</p>", "text/html")
-        message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
+        # Resend does not support attachments in batch send (2024-11-21)
+        # message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
 
         message.send()
         # Resend always queues: