fix(openai): wire local observer service tier in WS ingress; bound handler tests

- openai_ws_forwarder_ingress: resolve billing tier from the local
  upstreamResponseModelObserver (upstream echo first) instead of the raw
  request payload, matching the HTTP->WS bridge and WS v2 forwarder.
- openai_ws_http_bridge_test: add fast-alias + upstream default case
  proving the local observer's echoed tier wins.
- handler tests: keep only invalid service_tier -> 400 (short-circuits at
  validation); valid/omitted semantics covered by the pure service-level
  validation tests, avoiding real account selection in tests.

backend/internal/handler/openai_service_tier_validation_test.go

@@ -14,7 +14,12 @@ import (
 )
 
 // 非法 service_tier 必须在两个 OpenAI 端点（/v1/responses、/v1/chat/completions）
-// 上以 OpenAI 兼容错误结构返回 HTTP 400；合法值（fast/priority）不被拒绝。
+// 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
+// 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。
 
 func newServiceTierHandlerTest(t *testing.T) *OpenAIGatewayHandler {
 	t.Helper()
@@ -72,25 +77,6 @@ func TestOpenAIGatewayHandlerResponses_InvalidServiceTierRejected400(t *testing.
 	}
 }
 
-func TestOpenAIGatewayHandlerResponses_ValidServiceTierNotRejected(t *testing.T) {
-	for _, tier := range []string{"fast", "priority", "flex"} {
-		body := `{"model":"gpt-5.5","input":"hi","service_tier":"` + tier + `"}`
-		rec := runOpenAIHandlerServiceTierTest(t, "/v1/responses", body, func(h *OpenAIGatewayHandler, c *gin.Context) {
-			h.Responses(c)
-		})
-		require.NotEqual(t, http.StatusBadRequest, rec.Code, "tier=%s must not be rejected as invalid", tier)
-		require.NotContains(t, rec.Body.String(), "invalid service_tier", "tier=%s", tier)
-	}
-}
-
-func TestOpenAIGatewayHandlerResponses_ServiceTierOmittedKeepsCurrentBehavior(t *testing.T) {
-	body := `{"model":"gpt-5.5","input":"hi"}`
-	rec := runOpenAIHandlerServiceTierTest(t, "/v1/responses", body, func(h *OpenAIGatewayHandler, c *gin.Context) {
-		h.Responses(c)
-	})
-	require.NotContains(t, rec.Body.String(), "invalid service_tier")
-}
-
 func TestOpenAIGatewayHandlerChatCompletions_InvalidServiceTierRejected400(t *testing.T) {
 	for _, body := range []string{
 		`{"model":"gpt-5.5","messages":[{"role":"user","content":"hi"}],"service_tier":"turbo"}`,
@@ -106,22 +92,3 @@ func TestOpenAIGatewayHandlerChatCompletions_InvalidServiceTierRejected400(t *te
 		require.Contains(t, rec.Body.String(), "invalid service_tier", "body=%s", body)
 	}
 }
-
-func TestOpenAIGatewayHandlerChatCompletions_ValidServiceTierNotRejected(t *testing.T) {
-	for _, tier := range []string{"fast", "priority", "auto", "default", "scale", "flex"} {
-		body := `{"model":"gpt-5.5","messages":[{"role":"user","content":"hi"}],"service_tier":"` + tier + `"}`
-		rec := runOpenAIHandlerServiceTierTest(t, "/v1/chat/completions", body, func(h *OpenAIGatewayHandler, c *gin.Context) {
-			h.ChatCompletions(c)
-		})
-		require.NotEqual(t, http.StatusBadRequest, rec.Code, "tier=%q must not be rejected as invalid", tier)
-		require.NotContains(t, rec.Body.String(), "invalid service_tier", "tier=%q", tier)
-	}
-}
-
-func TestOpenAIGatewayHandlerChatCompletions_ServiceTierOmittedKeepsCurrentBehavior(t *testing.T) {
-	body := `{"model":"gpt-5.5","messages":[{"role":"user","content":"hi"}]}`
-	rec := runOpenAIHandlerServiceTierTest(t, "/v1/chat/completions", body, func(h *OpenAIGatewayHandler, c *gin.Context) {
-		h.ChatCompletions(c)
-	})
-	require.NotContains(t, rec.Body.String(), "invalid service_tier")
-}

backend/internal/service/openai_ws_forwarder_ingress.go

@@ -1164,7 +1164,7 @@ func (s *OpenAIGatewayService) ProxyResponsesWebSocketFromClient(
 					UpstreamResponseModel:         responseModelObserver.Model(),
 					UpstreamResponseModelConflict: responseModelObserver.Conflict(),
 					UpstreamResponseServiceTier:   responseModelObserver.ServiceTier(),
-					ServiceTier:                   extractOpenAIServiceTierFromBody(payload),
+					ServiceTier:                   resolvedOpenAIUpstreamServiceTierFromObserver(responseModelObserver, extractOpenAIServiceTierFromBody(payload)),
 					ReasoningEffort:               ApplyThinkingEnabledFallback(extractOpenAIReasoningEffortFromBody(payload, mappedModel, originalModel), payload, mappedModel),
 					Stream:                        reqStream,
 					OpenAIWSMode:                  true,

backend/internal/service/openai_ws_http_bridge_test.go

@@ -83,6 +83,43 @@ func TestProxyOpenAIWSHTTPBridgeTurn_UpstreamDefaultServiceTierWinsOverRequest(t
 	require.Equal(t, "default", *result.ServiceTier)
 }
 
+func TestProxyOpenAIWSHTTPBridgeTurn_UpstreamDefaultWinsOverFastAlias(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	// 客户端别名 fast 同样被上游回显的 default 覆盖：局部 observer 的
+	// ServiceTier() 是唯一计费依据，绝不回退到请求侧 fast。
+	sse := strings.Join([]string{
+		`data: {"type":"response.completed","response":{"id":"resp_tier2","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`,
+		``,
+	}, "\n")
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+		Body:       io.NopCloser(strings.NewReader(sse)),
+	}}
+	svc := &OpenAIGatewayService{
+		cfg:          &config.Config{Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize}},
+		httpUpstream: upstream,
+	}
+	account := &Account{ID: 5882, Platform: PlatformOpenAI, Type: AccountTypeAPIKey, Concurrency: 1}
+	payload := []byte(`{"type":"response.create","model":"gpt-5.5","stream":true,"service_tier":"fast","input":"hi"}`)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodGet, "/v1/responses", nil)
+
+	result, err := svc.proxyOpenAIWSHTTPBridgeTurn(
+		context.Background(), c, account, "test-token", payload, len(payload),
+		"gpt-5.5", "", "", "", "", 1,
+		func([]byte) error { return nil },
+	)
+
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.NotNil(t, result.ServiceTier)
+	require.Equal(t, "default", *result.ServiceTier,
+		"local observer's upstream-echoed default must win over the fast alias")
+}
+
 func TestProxyOpenAIWSHTTPBridgeTurnAPIKeyAdaptsClientTools(t *testing.T) {
 	gin.SetMode(gin.TestMode)