Follow-up to #116. MiniMax exposed only TARGET_DEPLOYMENT, and
set_optimizer_deployment() never touched MiniMax, so a configured
optimizer_model was ignored: chat_optimizer and the optimizer message path
both sent TARGET_DEPLOYMENT. In mixed optimizer/target setups this could send
the wrong model name to the optimizer endpoint.
Adds OPTIMIZER_DEPLOYMENT and set_optimizer_deployment() to minimax_backend,
wired into the model facade. chat_optimizer and a new chat_optimizer_messages
use it, falling back to TARGET_DEPLOYMENT when unset. Also fixes dict[int]
return annotations to dict[str, int]. Adds routing tests.
Co-Authored-By: Claude <noreply@anthropic.com>
Yif Yang