Merge tag 'v0.1.78' into develop

Resolve conflict in antigravity_gateway_service.go by keeping both
retry strategies:
- MODEL_CAPACITY_EXHAUSTED: handleModelCapacityExhaustedRetry (ours)
- Single-account 503 long delay: handleSingleAccountRetryInPlace (upstream)

Update tests to reflect that MODEL_CAPACITY_EXHAUSTED always goes
through capacity retry regardless of single-account mode.

backend/internal/handler/gemini_v1beta_handler.go

@@ -327,6 +327,13 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	var lastFailoverErr *service.UpstreamFailoverError
 	var forceCacheBilling bool // 粘性会话切换时的缓存计费标记
 
+	// 单账号分组提前设置 SingleAccountRetry 标记，让 Service 层首次 503 就不设模型限流标记。
+	// 避免单账号分组收到 503 (MODEL_CAPACITY_EXHAUSTED) 时设 29s 限流，导致后续请求连续快速失败。
+	if h.gatewayService.IsSingleAntigravityAccountGroup(c.Request.Context(), apiKey.GroupID) {
+		ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
+		c.Request = c.Request.WithContext(ctx)
+	}
+
 	for {
 		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, failedAccountIDs, "") // Gemini 不使用会话限制
 		if err != nil {
@@ -334,6 +341,19 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				googleError(c, http.StatusServiceUnavailable, "No available Gemini accounts: "+err.Error())
 				return
 			}
+			// Antigravity 单账号退避重试：分组内没有其他可用账号时，
+			// 对 503 错误不直接返回，而是清除排除列表、等待退避后重试同一个账号。
+			// 谷歌上游 503 (MODEL_CAPACITY_EXHAUSTED) 通常是暂时性的，等几秒就能恢复。
+			if lastFailoverErr != nil && lastFailoverErr.StatusCode == http.StatusServiceUnavailable && switchCount <= maxAccountSwitches {
+				if sleepAntigravitySingleAccountBackoff(c.Request.Context(), switchCount) {
+					log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d", switchCount, maxAccountSwitches)
+					failedAccountIDs = make(map[int64]struct{})
+					// 设置 context 标记，让 Service 层预检查等待限流过期而非直接切换
+					ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
+					c.Request = c.Request.WithContext(ctx)
+					continue
+				}
+			}
 			h.handleGeminiFailoverExhausted(c, lastFailoverErr)
 			return
 		}