fix(openai): adapt service tier observation to upstream constraints

- cc_pipeline: observe Chat Completions chunks/bodies as untyped payloads
  (empty event type) so the upstream-echoed service_tier is trusted, matching
  the upstream constraint that only terminal events and untyped bodies report
  the actual processing tier.
- tests: add model field to terminal SSE frames (observation only triggers on
  model-bearing frames) and assert response.created tier echo is ignored.

backend/internal/service/openai_fast_service_tier_test.go

@@ -757,17 +757,24 @@ func TestUpstreamResponseModelObserver_ObservesServiceTier(t *testing.T) {
 	t.Parallel()
 
 	observer := &upstreamResponseModelObserver{}
+	// 上游约束：非终止且有类型的事件（response.created）回显的是请求档位而非
+	// 实际处理档位，忽略。
 	observer.ObserveOpenAI([]byte(`{"type":"response.created","response":{"model":"gpt-5.5","service_tier":"flex"}}`), "response.created")
-	require.Equal(t, "flex", observer.ServiceTier())
+	require.Empty(t, observer.ServiceTier())
 
-	// terminal 声明优先。
+	// terminal 声明（带 model 帧）优先。
 	observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"model":"gpt-5.5","service_tier":"default"}}`), "response.completed")
 	require.Equal(t, "default", observer.ServiceTier())
 
-	// Chat Completions 顶层 service_tier 同样可观察。
+	// Chat Completions 顶层 service_tier 按 untyped payload 观察（无 type 字段）。
 	ccObserver := &upstreamResponseModelObserver{}
-	ccObserver.ObserveOpenAI([]byte(`{"id":"chatcmpl-1","model":"gpt-5.5","service_tier":"priority","choices":[]}`), "chat.completion")
+	ccObserver.ObserveOpenAI([]byte(`{"id":"chatcmpl-1","model":"gpt-5.5","service_tier":"priority","choices":[]}`), "")
 	require.Equal(t, "priority", ccObserver.ServiceTier())
+
+	// 无 model 的帧不触发 tier 观察（上游约束：tier 声明必带 model）。
+	modelFree := &upstreamResponseModelObserver{}
+	modelFree.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"service_tier":"default"}}`), "response.completed")
+	require.Empty(t, modelFree.ServiceTier())
 }
 
 func TestResolvedOpenAIUpstreamServiceTier(t *testing.T) {
@@ -779,7 +786,7 @@ func TestResolvedOpenAIUpstreamServiceTier(t *testing.T) {
 		gin.SetMode(gin.TestMode)
 		c, _ := gin.CreateTestContext(nil)
 		observer := beginUpstreamResponseModelObservation(c)
-		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"service_tier":"default"}}`), "response.completed")
+		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"model":"gpt-5.5","service_tier":"default"}}`), "response.completed")
 
 		got := resolvedOpenAIUpstreamServiceTier(c, priority)
 		require.NotNil(t, got)
@@ -800,7 +807,7 @@ func TestResolvedOpenAIUpstreamServiceTier(t *testing.T) {
 		gin.SetMode(gin.TestMode)
 		c, _ := gin.CreateTestContext(nil)
 		observer := beginUpstreamResponseModelObservation(c)
-		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"service_tier":"fast"}}`), "response.completed")
+		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"model":"gpt-5.5","service_tier":"fast"}}`), "response.completed")
 
 		got := resolvedOpenAIUpstreamServiceTier(c, nil)
 		require.NotNil(t, got)
@@ -819,7 +826,7 @@ func TestResolvedOpenAIUpstreamServiceTier(t *testing.T) {
 
 	t.Run("local observer wins without gin context", func(t *testing.T) {
 		observer := &upstreamResponseModelObserver{}
-		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"service_tier":"default"}}`), "response.completed")
+		observer.ObserveOpenAI([]byte(`{"type":"response.completed","response":{"model":"gpt-5.5","service_tier":"default"}}`), "response.completed")
 
 		got := resolvedOpenAIUpstreamServiceTierFromObserver(observer, priority)
 		require.NotNil(t, got)

backend/internal/service/openai_gateway_cc_pipeline.go

@@ -274,8 +274,10 @@ func (s *OpenAIGatewayService) scanCCStream(
 			break
 		}
 		// 观察上游 CC chunk 回显的 model / service_tier（计费以回显为准）。
+		// CC chunk 无 type 字段，按 untyped payload 观察（上游约束：只有终止
+		// 事件与无类型 body 报告实际处理档位）。
 		if observer := upstreamResponseModelObserverFromContext(c); observer != nil {
-			observer.ObserveOpenAI([]byte(payload), "chat.completion.chunk")
+			observer.ObserveOpenAI([]byte(payload), "")
 		}
 
 		if u := extractCCStreamUsage(payload); u != nil {
@@ -337,8 +339,9 @@ func (s *OpenAIGatewayService) readCCUpstreamJSONResponse(
 		return nil, OpenAIUsage{}, fmt.Errorf("parse chat completions response: %w", err)
 	}
 	// 观察上游 CC JSON 回显的 model / service_tier（计费以回显为准）。
+	// CC JSON 无 type 字段，按 untyped payload 观察（上游约束）。
 	if observer := upstreamResponseModelObserverFromContext(c); observer != nil {
-		observer.ObserveOpenAI(respBody, "chat.completion")
+		observer.ObserveOpenAI(respBody, "")
 	}
 
 	usage := OpenAIUsage{}

backend/internal/service/openai_ws_forwarder_v2_test.go

@@ -50,7 +50,7 @@ func TestForwardOpenAIWSV2_UpstreamDefaultServiceTierWinsOverRequest(t *testing.
 
 			captureConn := &openAIWSCaptureConn{
 				events: [][]byte{
-					[]byte(`{"type":"response.completed","response":{"id":"resp_tier_v2","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`),
+					[]byte(`{"type":"response.completed","response":{"id":"resp_tier_v2","model":"gpt-5.5","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`),
 				},
 			}
 			captureDialer := &openAIWSCaptureDialer{conn: captureConn}

backend/internal/service/openai_ws_http_bridge_test.go

@@ -52,7 +52,7 @@ func TestProxyOpenAIWSHTTPBridgeTurn_UpstreamDefaultServiceTierWinsOverRequest(t
 	// policy。本测试只覆盖局部 observer：canonical 请求 priority 被上游
 	// response.completed service_tier=default 覆盖。
 	sse := strings.Join([]string{
-		`data: {"type":"response.completed","response":{"id":"resp_tier","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`,
+		`data: {"type":"response.completed","response":{"id":"resp_tier","model":"gpt-5.5","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`,
 		``,
 	}, "\n")
 	upstream := &httpUpstreamRecorder{resp: &http.Response{
@@ -89,7 +89,7 @@ func TestProxyOpenAIWSHTTPBridgeTurn_UpstreamDefaultWinsOverFastAlias(t *testing
 	// 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}}}`,
+		`data: {"type":"response.completed","response":{"id":"resp_tier2","model":"gpt-5.5","status":"completed","service_tier":"default","usage":{"input_tokens":1,"output_tokens":1}}}`,
 		``,
 	}, "\n")
 	upstream := &httpUpstreamRecorder{resp: &http.Response{