fix(usage): 修复活跃请求中成本字段为 0 时被错误置为 None 的问题

将 actual_cost 和 rate_multiplier 的判断条件从 `if value else None`
改为 `if value is not None else None`，确保数值 0 不会被误判为 False

src/services/usage/service.py

@@ -1780,8 +1780,8 @@ class UsageService:
                 "cache_creation_input_tokens": r.cache_creation_input_tokens,
                 "cache_read_input_tokens": r.cache_read_input_tokens,
                 "cost": float(r.total_cost_usd) if r.total_cost_usd else 0,
-                "actual_cost": float(r.actual_total_cost_usd) if r.actual_total_cost_usd else None,
-                "rate_multiplier": float(r.rate_multiplier) if r.rate_multiplier else None,
+                "actual_cost": float(r.actual_total_cost_usd) if r.actual_total_cost_usd is not None else None,
+                "rate_multiplier": float(r.rate_multiplier) if r.rate_multiplier is not None else None,
                 "response_time_ms": r.response_time_ms,
                 "first_byte_time_ms": r.first_byte_time_ms,  # 首字时间 (TTFB)
             }