fix(BifrostAI): allow empty service tier strings

Author: Gaël Philippe

Sources/OpenAI/Public/Models/Types/ServiceTier.swift

@@ -8,8 +8,9 @@
 import Foundation
 
 public enum ServiceTier: String, Codable, Hashable, Sendable, CaseIterable {
-    case auto = "auto"
+    case auto
     case defaultTier = "default"
     case flexTier = "flex"
     case onDemand = "on_demand"
+    case empty = ""
 }