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).

When no vendor qualifies, one log line names the model and, for each vendor considered, the credential it would need (DescribeRealtimeVendorKeyNeeds) — a host whose LLM key works but whose realtime driver class has none otherwise fails with no trace of why.