fix(openai): include proxy host in upstream_failover_switching logs

Enrich openai.upstream_failover_switching structured logs with proxy
identity fields (proxy_id, proxy_name, proxy_host, proxy_port) so
operators can see which outbound proxy failed when switching accounts.
Matches the existing gateway.forward_failed / websocket_proxy_failed
logging pattern.

backend/internal/handler/openai_gateway_handler.go

@@ -479,12 +479,23 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 						h.handleFailoverExhausted(c, failoverErr, streamStarted)
 						return
 					}
-					reqLog.Warn("openai.upstream_failover_switching",
+					failoverSwitchFields := []zap.Field{
 						zap.Int64("account_id", account.ID),
 						zap.Int("upstream_status", failoverErr.StatusCode),
 						zap.Int("switch_count", switchCount),
 						zap.Int("max_switches", maxAccountSwitches),
-					)
+					}
+					if account.Proxy != nil {
+						failoverSwitchFields = append(failoverSwitchFields,
+							zap.Int64("proxy_id", account.Proxy.ID),
+							zap.String("proxy_name", account.Proxy.Name),
+							zap.String("proxy_host", account.Proxy.Host),
+							zap.Int("proxy_port", account.Proxy.Port),
+						)
+					} else if account.ProxyID != nil {
+						failoverSwitchFields = append(failoverSwitchFields, zap.Int64p("proxy_id", account.ProxyID))
+					}
+					reqLog.Warn("openai.upstream_failover_switching", failoverSwitchFields...)
 					continue
 				}
 				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)