feat(llm): fall back to models from another provider

[msmakouz]

What was changed

Model cards take fallback (model names or class: refs) and fallback_on (error types that switch; default: rate_limit_exceeded, server_error, timeout_error, network_error). generate/structured_output/evaluate try the card's routes by priority, then the fallbacks, under options.deadline_ms; max_tokens is capped to the candidate's output_tokens and reported in metadata.adjusted. Streams switch only before the first chunk reaches the client (stream_started). The answering route is returned in metadata.route / metadata.fallbacks; options.route pins it, and agent:step returns route_pin so hosts can keep it for the rest of a turn.