Route all Muse Spark models (not just 1.2) on OpenCode Free to
/zen/v1/responses via isMuseSparkModel(), fixing HTTP 500 on
muse-spark-1.3-contributor-free. Declare vision:true on Muse Spark
models so image input is no longer stripped; register 1.3 in the
registry and capabilities. Scoped to opencode only — other providers
keep Chat Completions routing.
muse-spark-1.2-contributor-free returned HTTP 500 on /zen/v1/chat/completions.
The model is only served by /zen/v1/responses, so route it there via a per-model
targetFormat and normalize the Chat fields the Responses API rejects
(max_tokens -> max_output_tokens, reasoning_effort -> reasoning{effort,summary}),
clamping max/ultra down to the highest effort the model accepts (xhigh).
Routing stays per-model: the other free models (big-pickle, hy3-free, mimo,
nemotron, laguna) are not served by /responses and keep /chat/completions.
