fix: stop passing est_tokens off as measured usage

usage 帧缺失/非法时不再拿 est_tokens(最坏情形上界)当实测值,chat 与
embedding 两处兜底改记 0 并标 unavailable;打捞覆盖加 measured 前置条件,
避免 0/0 被洗成 estimated 而算出假的 0.0。embedding 全批合并扩三态(任一批
不可得 → 整体不可得),_total_cost 遇不可得批整体记 NULL。

src/polygateway/types.py

@@ -284,7 +284,7 @@ class EmbeddingTransportResult:
     vectors: list[list[float]]
     dim: int
     prompt_tokens: int
-    usage_source: str  # measured | estimated
+    usage_source: str  # measured | estimated | unavailable
     raw: dict[str, Any]