fix(provider): classify HTTP 404 as model error for fallback

When a model returns 404 NOT_FOUND, treat it as FailoverModel so the
fallback chain continues to the next model instead of failing completely.

This fixes the issue where cerebras/gpt-oss-120b returning 404 was not
triggering fallback to other models.

pkg/providers/error_classifier.go

@@ -175,6 +175,8 @@ func classifyByStatus(status int) FailoverReason {
 		return FailoverAuth
 	case status == 402:
 		return FailoverBilling
+	case status == 404:
+		return FailoverModel // Model not found
 	case status == 408:
 		return FailoverTimeout
 	case status == 429: