feat: smart retry max 1 attempt + clear sticky session on failure

- Change antigravitySmartRetryMaxAttempts from 3 to 1 to prevent
  repeated rate limiting and long waits
- Clear sticky session binding (DeleteSessionAccountID) after smart
  retry exhaustion, so subsequent requests don't hit the same
  rate-limited account
- Add flow diagrams to Forward/ForwardGemini doc comments
- Add comprehensive unit tests covering:
  - Sticky session cleared on retry failure (429, 503, network error)
  - Sticky session NOT cleared on retry success
  - Sticky session NOT cleared for non-sticky requests (empty hash)
  - Sticky session NOT cleared on long delay path (handled by handler)
  - Nil cache safety (no panic)
  - MaxAttempts constant verification
  - End-to-end retryLoop → switchError propagation with session clear

backend/internal/service/antigravity_gateway_service.go

@@ -35,7 +35,7 @@ const (
 	// - 预检查：剩余限流时间 < 此阈值时等待，>= 此阈值时切换账号
 	antigravityRateLimitThreshold       = 7 * time.Second
 	antigravitySmartRetryMinWait        = 1 * time.Second  // 智能重试最小等待时间
-	antigravitySmartRetryMaxAttempts    = 3                // 智能重试最大次数
+	antigravitySmartRetryMaxAttempts    = 1                // 智能重试最大次数（仅重试 1 次，防止重复限流/长期等待）
 	antigravityDefaultRateLimitDuration = 30 * time.Second // 默认限流时间（无 retryDelay 时使用）
 
 	// Google RPC 状态和类型常量
@@ -247,6 +247,11 @@ func (s *AntigravityGatewayService) handleSmartRetry(p antigravityRetryLoopParam
 			}
 		}
 
+		// 清除粘性会话绑定，避免下次请求仍命中限流账号
+		if s.cache != nil && p.sessionHash != "" {
+			_ = s.cache.DeleteSessionAccountID(p.ctx, p.groupID, p.sessionHash)
+		}
+
 		// 返回账号切换信号，让上层切换账号重试
 		return &smartRetryResult{
 			action: smartRetryActionBreakWithResp,
@@ -952,6 +957,16 @@ func isModelNotFoundError(statusCode int, body []byte) bool {
 }
 
 // Forward 转发 Claude 协议请求（Claude → Gemini 转换）
+//
+// 限流处理流程:
+//
+//	请求 → antigravityRetryLoop → 预检查(remaining>0? → 切换账号) → 发送上游
+//	  ├─ 成功 → 正常返回
+//	  └─ 429/503 → handleSmartRetry
+//	      ├─ retryDelay >= 7s → 设置模型限流 + 清除粘性绑定 → 切换账号
+//	      └─ retryDelay <  7s → 等待后重试 1 次
+//	          ├─ 成功 → 正常返回
+//	          └─ 失败 → 设置模型限流 + 清除粘性绑定 → 切换账号
 func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context, account *Account, body []byte, isStickySession bool) (*ForwardResult, error) {
 	startTime := time.Now()
 	sessionID := getSessionID(c)
@@ -1571,6 +1586,16 @@ func stripSignatureSensitiveBlocksFromClaudeRequest(req *antigravity.ClaudeReque
 }
 
 // ForwardGemini 转发 Gemini 协议请求
+//
+// 限流处理流程:
+//
+//	请求 → antigravityRetryLoop → 预检查(remaining>0? → 切换账号) → 发送上游
+//	  ├─ 成功 → 正常返回
+//	  └─ 429/503 → handleSmartRetry
+//	      ├─ retryDelay >= 7s → 设置模型限流 + 清除粘性绑定 → 切换账号
+//	      └─ retryDelay <  7s → 等待后重试 1 次
+//	          ├─ 成功 → 正常返回
+//	          └─ 失败 → 设置模型限流 + 清除粘性绑定 → 切换账号
 func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Context, account *Account, originalModel string, action string, stream bool, body []byte, isStickySession bool) (*ForwardResult, error) {
 	startTime := time.Now()
 	sessionID := getSessionID(c)