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/service/openai_compact_probe.go

@@ -0,0 +1,120 @@
+package service
+
+import (
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	// AccountTestModeDefault drives the standard /responses connection test.
+	AccountTestModeDefault = "default"
+	// AccountTestModeCompact drives the /responses/compact compact-probe test.
+	AccountTestModeCompact = "compact"
+)
+
+func normalizeAccountTestMode(mode string) string {
+	switch strings.ToLower(strings.TrimSpace(mode)) {
+	case AccountTestModeCompact:
+		return AccountTestModeCompact
+	default:
+		return AccountTestModeDefault
+	}
+}
+
+func createOpenAICompactProbePayload(model string) map[string]any {
+	return map[string]any{
+		"model":        strings.TrimSpace(model),
+		"instructions": "You are a helpful coding assistant.",
+		"input": []any{
+			map[string]any{
+				"type":    "message",
+				"role":    "user",
+				"content": "Respond with OK.",
+			},
+		},
+	}
+}
+
+func shouldMarkOpenAICompactUnsupported(status int, body []byte) bool {
+	switch status {
+	case http.StatusNotFound, http.StatusMethodNotAllowed, http.StatusNotImplemented:
+		return true
+	case http.StatusBadRequest, http.StatusForbidden, http.StatusUnprocessableEntity:
+		lower := strings.ToLower(strings.TrimSpace(extractUpstreamErrorMessage(body) + " " + string(body)))
+		if strings.Contains(lower, "compact") {
+			for _, keyword := range []string{
+				"unsupported",
+				"not support",
+				"does not support",
+				"not available",
+				"disabled",
+			} {
+				if strings.Contains(lower, keyword) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func buildOpenAICompactProbeExtraUpdates(resp *http.Response, body []byte, probeErr error, now time.Time) map[string]any {
+	updates := map[string]any{
+		"openai_compact_checked_at":  now.Format(time.RFC3339),
+		"openai_compact_last_status": nil,
+	}
+
+	if resp != nil {
+		updates["openai_compact_last_status"] = resp.StatusCode
+	}
+
+	switch {
+	case probeErr != nil:
+		updates["openai_compact_last_error"] = truncateString(sanitizeUpstreamErrorMessage(probeErr.Error()), 2048)
+	case resp == nil:
+		updates["openai_compact_last_error"] = "compact probe failed"
+	default:
+		errMsg := strings.TrimSpace(extractUpstreamErrorMessage(body))
+		if errMsg == "" && len(body) > 0 {
+			errMsg = strings.TrimSpace(string(body))
+		}
+		if errMsg == "" && (resp.StatusCode < 200 || resp.StatusCode >= 300) {
+			errMsg = "HTTP " + strconv.Itoa(resp.StatusCode)
+		}
+		errMsg = truncateString(sanitizeUpstreamErrorMessage(errMsg), 2048)
+		if resp.StatusCode >= 200 && resp.StatusCode < 300 {
+			updates["openai_compact_supported"] = true
+			updates["openai_compact_last_error"] = ""
+		} else {
+			if shouldMarkOpenAICompactUnsupported(resp.StatusCode, body) {
+				updates["openai_compact_supported"] = false
+			}
+			updates["openai_compact_last_error"] = errMsg
+		}
+	}
+
+	return updates
+}
+
+func mergeExtraUpdates(base map[string]any, more map[string]any) map[string]any {
+	if len(base) == 0 && len(more) == 0 {
+		return nil
+	}
+	out := make(map[string]any, len(base)+len(more))
+	for key, value := range base {
+		out[key] = value
+	}
+	for key, value := range more {
+		out[key] = value
+	}
+	return out
+}
+
+func compactProbeSessionID(accountID int64) string {
+	if accountID <= 0 {
+		return "probe_compact"
+	}
+	return "probe_compact_" + strconv.FormatInt(accountID, 10)
+}