feat(codex): support ultrafast service tier

Accept service_tier=ultrafast on the OpenAI/Codex gateway, advertise it
on GPT-5.6 Sol in the Codex model catalog, and include it in ChatGPT
OAuth routing hints so Codex clients can select Ultrafast.

backend/internal/handler/admin/admin_helpers_test.go

@@ -278,13 +278,15 @@ func TestOpenAIFastPolicySettingsFromDTO_NormalizesServiceTier(t *testing.T) {
 			Rules: []dto.OpenAIFastPolicyRule{
 				{ServiceTier: "priority", Action: "filter", Scope: "all"},
 				{ServiceTier: "flex", Action: "block", Scope: "oauth"},
+				{ServiceTier: "ultrafast", Action: "pass", Scope: "all"},
 				{ServiceTier: "all", Action: "pass", Scope: "apikey"},
 			},
 		}
 		out := openaiFastPolicySettingsFromDTO(in)
-		require.Len(t, out.Rules, 3)
+		require.Len(t, out.Rules, 4)
 		require.Equal(t, service.OpenAIFastTierPriority, out.Rules[0].ServiceTier)
 		require.Equal(t, service.OpenAIFastTierFlex, out.Rules[1].ServiceTier)
-		require.Equal(t, service.OpenAIFastTierAny, out.Rules[2].ServiceTier)
+		require.Equal(t, service.OpenAIFastTierUltrafast, out.Rules[2].ServiceTier)
+		require.Equal(t, service.OpenAIFastTierAny, out.Rules[3].ServiceTier)
 	})
 }

backend/internal/handler/openai_service_tier_validation_test.go

@@ -17,7 +17,7 @@ import (
 // 上以 OpenAI 兼容错误结构返回 HTTP 400。这些用例在 handler 的 service_tier
 // 校验处短路，不会进入账号选择/重试。
 //
-// 合法值（fast/priority/flex/auto/default/scale）与省略/null 的接受语义由
+// 合法值（fast/priority/flex/auto/default/scale/ultrafast）与省略/null 的接受语义由
 // service 层纯校验函数 TestValidateOpenAIServiceTierField 覆盖，避免 handler
 // 测试走入真实账号选择/重试路径。