Function SelectRealtimeVendorForModel

Selects the vendor that will run a realtime model: the highest-Priority Active MJ: AI Model Vendors row for the model whose DriverClass resolves an API key.

Vendors are walked in descending Priority and the FIRST with a usable key wins — a keyless higher-priority vendor is skipped rather than dead-ending the resolution (a deployment that configures only one provider's key must still resolve that provider).