refactor: 抽取统一计费模块，支持配置驱动的多厂商计费

- 新增 src/services/billing/ 模块，包含计费计算器、模板和使用量映射
- 将 ChatAdapterBase 和 CliAdapterBase 中的计费逻辑重构为调用 billing 模块
- 为每个 adapter 添加 BILLING_TEMPLATE 类属性，指定计费模板
- 支持 Claude/OpenAI/Gemini 三种计费模板，支持阶梯计费和缓存 TTL 定价
- 新增 tests/services/billing/ 单元测试

src/services/billing/__init__.py

@@ -0,0 +1,51 @@
+"""
+计费模块
+
+提供配置驱动的计费计算，支持不同厂商的差异化计费模式：
+- Claude: input + output + cache_creation + cache_read
+- OpenAI: input + output + cache_read (无缓存创建费用)
+- 豆包: input + output + cache_read + cache_storage (缓存按时计费)
+- 按次计费: per_request
+
+使用方式:
+    from src.services.billing import BillingCalculator, UsageMapper, StandardizedUsage
+
+    # 1. 将原始 usage 映射为标准格式
+    usage = UsageMapper.map(raw_usage, api_format="OPENAI")
+
+    # 2. 使用计费计算器计算费用
+    calculator = BillingCalculator(template="openai")
+    result = calculator.calculate(usage, prices)
+
+    # 3. 获取费用明细
+    print(result.total_cost)
+    print(result.costs)  # {"input": 0.01, "output": 0.02, ...}
+"""
+
+from src.services.billing.calculator import BillingCalculator, calculate_request_cost
+from src.services.billing.models import (
+    BillingDimension,
+    BillingUnit,
+    CostBreakdown,
+    StandardizedUsage,
+)
+from src.services.billing.templates import BILLING_TEMPLATE_REGISTRY, BillingTemplates
+from src.services.billing.usage_mapper import UsageMapper, map_usage, map_usage_from_response
+
+__all__ = [
+    # 数据模型
+    "BillingDimension",
+    "BillingUnit",
+    "CostBreakdown",
+    "StandardizedUsage",
+    # 模板
+    "BillingTemplates",
+    "BILLING_TEMPLATE_REGISTRY",
+    # 计算器
+    "BillingCalculator",
+    "calculate_request_cost",
+    # 映射器
+    "UsageMapper",
+    "map_usage",
+    "map_usage_from_response",
+]