DeepSeek's API accepts reasoning_effort low/high/max and only
deepseek-v4-flash supports all three tiers (V4 Pro is high/max). The
identity-derived effort fallback blanket-applied high/max to every
direct-DeepSeek reasoning model, hiding the low tier flash accepts.
Added isDeepseekV4FlashModelId and route flash to the low/high/max ladder
on every host; non-flash DeepSeek keeps high/max (high-only on OpenRouter).
Fixes#7668