test(linkai): cover the midjourney reply retry path

The midjourney plugin had no tests for _send. Cover the behaviour the
retry loop is responsible for: a send that fails twice and then succeeds
must be retried with the 3s/6s backoff, a permanently failing send must
give up after two retries without raising, and NotImplementedError must
still short-circuit. The first two fail with

    TypeError: send() takes 3 positional arguments but 4 were given

before the fix.

Follows tests/test_keyword_attachment_urls.py for loading a plugin module.