arch: rm support for init'ing attempt.prompt with str

[leondz]

fixes #1347

attempt.prompt accepts Message or Conversation - now we check for this

hey seeing as local models are difficult to run, are perhaps the minority case, and are where most of our deps come from, what are thoughts on dropping these, and migrating to something strongly typed, like rust?

Verification

• tests pass
• garak -m test -g 1 runs