Revert two overreaches from #13699 that forced paid Nous vision to
xiaomi/mimo-v2-omni instead of the tier-appropriate gemini-3-flash-preview:
1. Remove "nous": "xiaomi/mimo-v2-omni" from _PROVIDER_VISION_MODELS —
#13696 already routes nous main-provider vision through the strict
backend, and this entry caused any direct resolve_provider_client(
"nous", ...) aggregator-lookup path to pick the wrong model for paid.
2. Drop the 'elif vision' paid override in _try_nous() that forced
mimo-v2-omni on every Nous vision call regardless of tier. Paid
accounts now keep gemini-3-flash-preview for vision as well as text.
Free-tier behavior unchanged: still uses mimo-v2-omni for vision,
mimo-v2-pro for text (check_nous_free_tier() branch).
E2E verified:
paid vision → google/gemini-3-flash-preview
free vision → xiaomi/mimo-v2-omni
paid text → google/gemini-3-flash-preview
free text → xiaomi/mimo-v2-pro
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