feat: add derived field support for nested usage data

- Support dot notation in derived expressions (e.g., prompt_tokens_details.cached_tokens)
- Auto-flatten nested dict keys to underscore format for eval
- Calculate uncached_prompt_tokens and cached_tokens from raw usage data
- Test case: qwen3.7-max pricing with derived fields

Test result:
- uncached_prompt_tokens = prompt_tokens - cached_tokens = 1075
- Total cost: 0.007116 元 ✓

pricing/pricing.py

@@ -529,6 +529,9 @@ order by b.enabled_date desc"""
 		支持两种格式：
 		1. 旧格式：formula 字段（eval计算）
 		2. 新格式：price_factors + unit_prices + unit（自动计算）
+		
+		支持 derived 字段：在 fields 中定义 derived 表达式，从原始 usage 数据计算衍生字段
+		例如：uncached_prompt_tokens.derived = "prompt_tokens - prompt_tokens_details.cached_tokens"
 		"""
 		if config_data is None:
 			e = Exception(f'config_data is None, {yamlstr=}')
@@ -549,6 +552,34 @@ order by b.enabled_date desc"""
 			exception(f'{d} has not "pricings"')
 			raise Exception(f'定价定义中没有pricing数据')
 		
+		# 处理 derived 字段：从原始 usage 数据计算衍生字段
+		for field_name, field_def in d.fields.items():
+			if not isinstance(field_def, dict):
+				continue
+			derived_expr = field_def.get('derived')
+			if not derived_expr:
+				continue
+			
+			# 构建 eval 环境：将 config_data 中的所有字段和嵌套字段展平
+			# dot notation 转为下划线 key（Python eval 不支持带 dot 的变量名）
+			eval_env = {}
+			for k, v in config_data.items():
+				eval_env[k] = v
+				if isinstance(v, dict):
+					for sub_k, sub_v in v.items():
+						eval_env[f'{k}_{sub_k}'] = sub_v
+			
+			# 将表达式中的 dot 替换为下划线
+			processed_expr = derived_expr.replace('.', '_')
+			
+			try:
+				result = eval(processed_expr, {}, eval_env)
+				config_data[field_name] = result
+				debug(f'derived field {field_name} = {derived_expr} = {result}')
+			except Exception as e:
+				debug(f'derived field {field_name} evaluation failed: {derived_expr}, error: {e}')
+				config_data[field_name] = 0
+		
 		# 单位映射表
 		unit_values = d.get('unit_values', {'百万': 1000000, '秒': 1, '千': 1000, '次': 1, '张': 1, '毫秒': 0.001, '元/百万tokens': 1000000, '元/total_tokens': 1, '元/times': 1})