feat: add API key load balancing for providers (#676)

Support multiple API keys per provider with random selection for load
balancing. When AI_MODELS_CONFIG has multiple apiKeyEnv values for
a provider, requests will randomly select one available key.

- Update schema to accept apiKeyEnv as string or string array
- Add random key selection in resolveApiKey()
- Update validation to check at least one key exists
- Add tests for array format support

app/api/chat/route.ts

@@ -186,7 +186,7 @@ async function handleChatRequest(req: Request): Promise<Response> {
 
     // Check if this is a server model with custom env var names
     let serverModelConfig: {
-        apiKeyEnv?: string
+        apiKeyEnv?: string | string[]
         baseUrlEnv?: string
         provider?: string
     } = {}