deepseek-v4-flash / deepseek-v4-pro expose thinking-mode effort control
through the OpenAI-compatible reasoning_effort parameter (low/high/max;
medium/xhigh map onto high). reasoningEffortsForModelName did not recognize
the deepseek family, so /effort only offered 'auto' for these models.
Advertise {low, high, max} so the /effort picker and Ctrl+T ring work, and
add fallback coverage for deepseek-v4-flash/pro/reasoner/chat.