Adds OpenCode Zen and OpenCode Go as first-class LiteLLM providers,
each serving three wire formats: Chat Completions, Anthropic Messages,
and OpenAI Responses.
Routing between the three arms is decided in code, not from the runtime
cost map, because a published map predating this provider would send
every Messages-native model down the wrong wire. The model sets carry
the full forward-looking grid of model names so a gateway-side addition
routes correctly without a release; names without a bundled price simply
stay unpriced until a real one is published.
Cost resolution falls back to pricing bundled with the package when the
runtime cost map carries no usable entry -- the Router registers a bare
placeholder for every deployment at startup, so the guard asks for
pricing the cost calculator can actually use, not for the key's
presence.
The cost-map JSON schema gains a `messages` mode so the new entries
validate, and the provider tests set module-level configuration through
monkeypatch rather than writing process-wide globals directly.
