fix(gateway): WS 连接池条件式 MarkBroken 防止跨请求串流

正常终端事件（response.completed 等）退出后连接归还复用，
仅异常路径（读写错误、error 事件、客户端断连）MarkBroken 销毁。

Generate 模式:
- 引入 cleanExit 标记，仅在 isTerminalEvent break 时设置 true
- defer 中根据 cleanExit 决定是否 MarkBroken
- 所有异常路径已在各自分支中提前调用 MarkBroken

Ingress 模式:
- 引入 lastTurnClean 标记，sendAndRelay 正常完成时设为 true
- releaseSessionLease 根据 lastTurnClean 决定是否 MarkBroken
- 错误路径重置 lastTurnClean = false
- 客户端断连后 drain 仍保守 MarkBroken（L2916）

backend/internal/service/openai_ws_forwarder.go

@@ -1870,7 +1870,16 @@ func (s *OpenAIGatewayService) forwardOpenAIWSV2(
 		}
 		return nil, wrapOpenAIWSFallback(classifyOpenAIWSAcquireError(err), err)
 	}
-	defer lease.Release()
+	// cleanExit 标记正常终端事件退出，此时上游不会再发送帧，连接可安全归还复用。
+	// 所有异常路径（读写错误、error 事件等）已在各自分支中提前调用 MarkBroken，
+	// 因此 defer 中只需处理正常退出时不 MarkBroken 即可。
+	cleanExit := false
+	defer func() {
+		if !cleanExit {
+			lease.MarkBroken()
+		}
+		lease.Release()
+	}()
 	connID := strings.TrimSpace(lease.ConnID())
 	logOpenAIWSModeDebug(
 		"connected account_id=%d account_type=%s transport=%s conn_id=%s conn_reused=%v conn_pick_ms=%d queue_wait_ms=%d has_previous_response_id=%v",
@@ -2248,6 +2257,7 @@ func (s *OpenAIGatewayService) forwardOpenAIWSV2(
 		}
 
 		if isTerminalEvent {
+			cleanExit = true
 			break
 		}
 	}
@@ -2983,12 +2993,15 @@ func (s *OpenAIGatewayService) ProxyResponsesWebSocketFromClient(
 			pinnedSessionConnID = connID
 		}
 	}
+	// lastTurnClean 标记最后一轮 sendAndRelay 是否正常完成（收到终端事件且客户端未断连）。
+	// 所有异常路径（读写错误、error 事件、客户端断连）已在各自分支或上层（L3403）中 MarkBroken，
+	// 因此 releaseSessionLease 中只需在非正常结束时 MarkBroken。
+	lastTurnClean := false
 	releaseSessionLease := func() {
 		if sessionLease == nil {
 			return
 		}
-		if dedicatedMode {
-			// dedicated 会话结束后主动标记损坏，确保连接不会跨会话复用。
+		if !lastTurnClean {
 			sessionLease.MarkBroken()
 		}
 		unpinSessionConn(sessionConnID)
@@ -3383,6 +3396,7 @@ func (s *OpenAIGatewayService) ProxyResponsesWebSocketFromClient(
 
 		result, relayErr := sendAndRelay(turn, sessionLease, currentPayload, currentPayloadBytes, currentOriginalModel)
 		if relayErr != nil {
+			lastTurnClean = false
 			if recoverIngressPrevResponseNotFound(relayErr, turn, connID) {
 				continue
 			}
@@ -3402,6 +3416,7 @@ func (s *OpenAIGatewayService) ProxyResponsesWebSocketFromClient(
 		turnRetry = 0
 		turnPrevRecoveryTried = false
 		lastTurnFinishedAt = time.Now()
+		lastTurnClean = true
 		if hooks != nil && hooks.AfterTurn != nil {
 			hooks.AfterTurn(turn, result, nil)
 		}

backend/internal/service/openai_ws_forwarder_success_test.go

@@ -380,7 +380,8 @@ func TestOpenAIGatewayService_Forward_WSv2_PoolReuseNotOneToOne(t *testing.T) {
 		require.True(t, strings.HasPrefix(result.RequestID, "resp_reuse_"))
 	}
 
-	require.Equal(t, int64(1), upgradeCount.Load(), "多个客户端请求应复用账号连接池而不是 1:1 对等建链")
+	// 条件式 MarkBroken：正常终端事件退出后连接归还复用，不再无条件销毁。
+	require.Equal(t, int64(1), upgradeCount.Load(), "正常完成后连接应归还复用，不应每次新建")
 	metrics := svc.SnapshotOpenAIWSPoolMetrics()
 	require.GreaterOrEqual(t, metrics.AcquireReuseTotal, int64(1))
 	require.GreaterOrEqual(t, metrics.ConnPickTotal, int64(1))
@@ -964,6 +965,10 @@ func TestOpenAIGatewayService_Forward_WSv2_TurnMetadataInPayloadOnConnReuse(t *t
 	require.NotNil(t, result1)
 	require.Equal(t, "resp_meta_1", result1.RequestID)
 
+	require.Len(t, captureConn.writes, 1)
+	firstWrite := requestToJSONString(captureConn.writes[0])
+	require.Equal(t, "turn_meta_payload_1", gjson.Get(firstWrite, "client_metadata.x-codex-turn-metadata").String())
+
 	rec2 := httptest.NewRecorder()
 	c2, _ := gin.CreateTestContext(rec2)
 	c2.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/responses", nil)
@@ -977,7 +982,7 @@ func TestOpenAIGatewayService_Forward_WSv2_TurnMetadataInPayloadOnConnReuse(t *t
 	require.Equal(t, 1, captureDialer.DialCount(), "同一账号两轮请求应复用同一 WS 连接")
 	require.Len(t, captureConn.writes, 2)
 
-	firstWrite := requestToJSONString(captureConn.writes[0])
+	firstWrite = requestToJSONString(captureConn.writes[0])
 	secondWrite := requestToJSONString(captureConn.writes[1])
 	require.Equal(t, "turn_meta_payload_1", gjson.Get(firstWrite, "client_metadata.x-codex-turn-metadata").String())
 	require.Equal(t, "turn_meta_payload_2", gjson.Get(secondWrite, "client_metadata.x-codex-turn-metadata").String())