fix: 展平嵌套usage数据以支持pricing引擎点号路径查找

根因：pricing引擎的config_data.get(k)是平面dict查找，
当k='prompt_tokens_details.cached_tokens'时无法从嵌套结构取值。
在llm_charging()中将prompt_tokens_details和completion_tokens_details
的子键展平为顶层key（如'prompt_tokens_details.cached_tokens'）。

llmage/accounting.py

@@ -16,6 +16,13 @@ async def llm_charging(ppid, llmusage):
 	usages =  llmusage.usages
 	if isinstance(usages, str):
 		usages = json.loads(usages)
+	# 展平嵌套dict，使 prompt_tokens_details.cached_tokens 等点号路径
+	# 可以作为顶层key被pricing引擎的 config_data.get(k) 取到
+	for nested_key in ('prompt_tokens_details', 'completion_tokens_details'):
+		nested = usages.get(nested_key)
+		if isinstance(nested, dict):
+			for k, v in nested.items():
+				usages[f'{nested_key}.{k}'] = v
 	prices = await env.buffered_charging(ppid, usages)
 	if prices is None:
 		e = Exception(f'{ppid=}, {usages=}{llmusage.id=}  env.buffered_charging() return None')