fix: keep Replicate private endpoint output as one Message

[fallintoplace]

private Replicate endpoints already join prediction.output into one string, but the private endpoint path then iterates that string when building the return value.

so a response like "hello" becomes a list of per-character messages instead of a single Message("hello").

this PR:

• returns one Message for the joined response
• adds a regression test for chunked prediction.output

verification

• ran PYTHONPATH=/Users/hoangvu/Code/OSS/garak /tmp/garak-pr-venv/bin/python -m pytest tests/generators/test_replicate.py
• test passes with this change
• the same test fails against the old behavior because _call_model() returns one message per character

I didn't run a live private Replicate endpoint check here, and I didn't run the full tests/ suite in this local environment.