Switch default Omni model to DeepSeek-V4.1-Flash (#2580)
chore(router): switch default Omni model to deepseek-ai/DeepSeek-V4.1-Flash
V4.1-Flash becomes the primary model on the default, multimodal, and
agentic routes, and the multimodal and tools bypass env vars point at it
in prod and dev. Kimi-K2.6, the previous default, moves to first fallback
on every route ahead of the existing chain. The router docs sample is
refreshed to match.
