Xiaomi's API (api.xiaomimimo.com) requires lowercase model IDs like
"mimo-v2.5-pro" but rejects mixed-case names like "MiMo-V2.5-Pro"
that users copy from marketing docs or the ProviderEntry description.
Add _LOWERCASE_MODEL_PROVIDERS set and apply .lower() to model names
for providers in this set (currently just xiaomi) after stripping the
provider prefix. This ensures any case variant in config.yaml is
normalized before hitting the API.
Other providers (minimax, zai, etc.) are NOT affected — their APIs
accept mixed case (e.g. MiniMax-M2.7).
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