_OPENROUTER_MODEL hardcoded 'google/gemini-3-flash-preview' which
returns 404 on OpenRouter, breaking all vision tasks for users who
rely on the OpenRouter default. Additionally, _try_openrouter()
ignored the user-configured auxiliary.vision.model entirely.
Changes:
- Update _OPENROUTER_MODEL default to google/gemini-2.5-flash (valid)
- Add optional 'model' parameter to _try_openrouter()
- Pass configured model from _resolve_strict_vision_backend() through
to _try_openrouter()
This allows users who set auxiliary.vision.model (e.g. x-ai/grok-4.3)
to have it actually used, while maintaining backward compatibility.
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