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.
Resend's API no longer returns 403
responses for bad API keys (or 401
for missing auth header). Now, it just
never responds, leading to a TimeoutError.
Since this seems likely to be a Resend
bug, just disable the bad API key
integration test for now.
(Resend changed behavior sometime between
2023-12-07 and 2023-12-14, from our
scheduled integration test runs.)
