fix(gateway): harden digest logging and align antigravity ops

- avoid panic by using safe UUID prefix truncation in Gemini digest fallback logs\n- remove unconditional Antigravity 429 full-body debug logs and honor log truncation config\n- align Antigravity quick preset mappings to opus 4.6-thinking targets only\n- restore scope rate-limit aggregation/output in ops availability stats

backend/internal/handler/gemini_v1beta_handler.go

@@ -294,7 +294,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 					sessionBoundAccountID = foundAccountID
 					geminiSessionUUID = foundUUID
 					log.Printf("[Gemini] Digest fallback matched: uuid=%s, accountID=%d, chain=%s",
-						foundUUID[:8], foundAccountID, truncateDigestChain(geminiDigestChain))
+						safeShortPrefix(foundUUID, 8), foundAccountID, truncateDigestChain(geminiDigestChain))
 
 					// 关键：如果原 sessionKey 为空，使用 prefixHash + uuid 作为 sessionKey
 					// 这样 SelectAccountWithLoadAwareness 的粘性会话逻辑会优先使用匹配到的账号
@@ -650,6 +650,15 @@ func truncateDigestChain(chain string) string {
 	return chain[:50] + "..."
 }
 
+// safeShortPrefix 返回字符串前 n 个字符；长度不足时返回原字符串。
+// 用于日志展示，避免切片越界。
+func safeShortPrefix(value string, n int) string {
+	if n <= 0 || len(value) <= n {
+		return value
+	}
+	return value[:n]
+}
+
 // derefGroupID 安全解引用 *int64，nil 返回 0
 func derefGroupID(groupID *int64) int64 {
 	if groupID == nil {