feat(openai): port /responses/compact account support flow (PR #1555)

将 vansour/sub2api#1555 的 OpenAI compact 能力建模手工移植到当前 main：账号
级 compact 状态/auto-force_on-force_off 模式、compact-only 模型映射、调度器
tier 分层（已支持 > 未知 > 已知不支持）、管理后台 compact 主动探测，以及对应
i18n/状态徽章。普通 /responses 流量行为不变，无数据库迁移。

backend/internal/handler/admin/account_handler.go

@@ -652,6 +652,7 @@ func (h *AccountHandler) Delete(c *gin.Context) {
 type TestAccountRequest struct {
 	ModelID string `json:"model_id"`
 	Prompt  string `json:"prompt"`
+	Mode    string `json:"mode"`
 }
 
 type SyncFromCRSRequest struct {
@@ -682,7 +683,7 @@ func (h *AccountHandler) Test(c *gin.Context) {
 	_ = c.ShouldBindJSON(&req)
 
 	// Use AccountTestService to test the account with SSE streaming
-	if err := h.accountTestService.TestAccountConnection(c, accountID, req.ModelID, req.Prompt); err != nil {
+	if err := h.accountTestService.TestAccountConnection(c, accountID, req.ModelID, req.Prompt, req.Mode); err != nil {
 		// Error already sent via SSE, just log
 		return
 	}

backend/internal/handler/openai_chat_completions.go

@@ -130,6 +130,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 			reqModel,
 			failedAccountIDs,
 			service.OpenAIUpstreamTransportAny,
+			false,
 		)
 		if err != nil {
 			reqLog.Warn("openai_chat_completions.account_select_failed",
@@ -153,6 +154,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 						defaultModel,
 						failedAccountIDs,
 						service.OpenAIUpstreamTransportAny,
+						false,
 					)
 					if err == nil && selection != nil {
 						c.Set("openai_chat_completions_fallback_model", defaultModel)

backend/internal/handler/openai_gateway_handler.go

@@ -238,6 +238,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 
 	// Generate session hash (header first; fallback to prompt_cache_key)
 	sessionHash := h.gatewayService.GenerateSessionHash(c, sessionHashBody)
+	requireCompact := isOpenAIRemoteCompactPath(c)
 
 	maxAccountSwitches := h.maxAccountSwitches
 	switchCount := 0
@@ -256,6 +257,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 			reqModel,
 			failedAccountIDs,
 			service.OpenAIUpstreamTransportAny,
+			requireCompact,
 		)
 		if err != nil {
 			reqLog.Warn("openai.account_select_failed",
@@ -263,6 +265,10 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 				zap.Int("excluded_account_count", len(failedAccountIDs)),
 			)
 			if len(failedAccountIDs) == 0 {
+				if errors.Is(err, service.ErrNoAvailableCompactAccounts) {
+					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "compact_not_supported", "No available OpenAI accounts support /responses/compact", streamStarted)
+					return
+				}
 				h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
 				return
 			}
@@ -644,6 +650,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 			currentRoutingModel,
 			failedAccountIDs,
 			service.OpenAIUpstreamTransportAny,
+			false,
 		)
 		if err != nil {
 			reqLog.Warn("openai_messages.account_select_failed",
@@ -1167,6 +1174,7 @@ func (h *OpenAIGatewayHandler) ResponsesWebSocket(c *gin.Context) {
 		reqModel,
 		nil,
 		service.OpenAIUpstreamTransportResponsesWebsocketV2,
+		false,
 	)
 	if err != nil {
 		reqLog.Warn("openai.websocket_account_select_failed", zap.Error(err))