When the agent config stored a model alias (e.g. 'k2p5') instead of
the full qualified key (e.g. 'kimi-coding/k2p5'), buildModelOptions
would add the bare alias as an option value. The backend would then
resolve 'k2p5' using the default provider (minimax), producing
'minimax/k2p5' which is not in the allowed list.
Now buildModelOptions reverse-matches the alias to the full key before
using the value, ensuring the select always submits a complete
provider/key pair.
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