_make_agent() was not calling resolve_runtime_provider(), so bare-slug
models (e.g. 'claude-opus-4-6' with provider: anthropic) left provider,
base_url, and api_key empty in AIAgent — causing HTTP 404 at
api.anthropic.com.
Now mirrors cli.py: calls resolve_runtime_provider(requested=None) and
forwards all 7 resolved fields to AIAgent.
Adds regression test.
