Test Llm Config Inline

Run a one-shot test invocation against a fully inline LLM configuration that has not been persisted yet (the create flow), and return the model's reply.

Because there is no stored record to recover secrets from, the provider api_key must be supplied in body.config (or omitted to fall back to the team's stored vendor credentials). Provider/runtime failures are returned as HTTP 200 with success=False and a clean error message, mirroring the saved-config test endpoint, so the dashboard can render them inline.