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。

tests/unit/test_embedding.py

@@ -102,12 +102,14 @@ class TestEmbedTransport:
         assert result.dim == 2
         assert result.prompt_tokens == 5 and result.usage_source == "measured"
 
-    async def test_missing_usage_falls_back_estimated(self):
+    async def test_missing_usage_is_unavailable(self):
+        """usage 缺失不再退到 `est_tokens`(夹具填 7),与 chat 同口径记 0 + unavailable。"""
+
         def handler(request):
             return httpx.Response(200, json=_ok_body([[1.0]]))
 
         result = await _transport_with(handler).embed(texts=["a"], source=_src(), call_id="c")
-        assert result.prompt_tokens == 7 and result.usage_source == "estimated"  # est_tokens
+        assert result.prompt_tokens == 0 and result.usage_source == "unavailable"
 
     @pytest.mark.parametrize(
         ("status", "exc_type"),
@@ -161,6 +163,7 @@ from polygateway.backends.memory.breaker import InMemoryGate  # noqa: E402
 from polygateway.backends.memory.limiter import InMemoryLimiter  # noqa: E402
 from polygateway.config import EmbeddingSettings  # noqa: E402
 from polygateway.embedding import EmbeddingClient  # noqa: E402
+from polygateway.pricing import ModelPrice, PricingTable  # noqa: E402
 from polygateway.sources import RoundRobinSelector  # noqa: E402
 from polygateway.types import (  # noqa: E402
     BackpressurePolicy,
@@ -260,6 +263,28 @@ class TestEmbedBatching:
         assert resp.usage_source == "estimated"  # 任一批 estimated 则整体 estimated
         assert resp.prompt_tokens == 2 + 9
 
+    async def test_unavailable_batch_dominates_and_voids_cost(self):
+        """三态合并优先级(设计 §3.2 #10/#11): 任一批不可得 → 整体不可得且 cost NULL。
+
+        改前二值合并只看 `estimated`,measured+unavailable 会误标 measured;
+        `_total_cost` 逐批求和还会给出一个偏低却看似有效的金额。
+        """
+        estimated = EmbeddingTransportResult(
+            vectors=[[1.0], [1.0]], dim=1, prompt_tokens=9, usage_source="estimated", raw={}
+        )
+        unavailable = EmbeddingTransportResult(
+            vectors=[[1.0], [1.0]], dim=1, prompt_tokens=0, usage_source="unavailable", raw={}
+        )
+        client, _ = _embed_client(
+            [_src()],
+            ["ok", estimated, unavailable],
+            batch_size=2,
+            pricing=PricingTable({"embed-1": ModelPrice(input_per_1m=1.0, output_per_1m=0.0)}),
+        )
+        resp = await client.embed(["a", "b", "c", "d", "e", "f"])
+        assert resp.usage_source == "unavailable"  # unavailable 压过 estimated 与 measured
+        assert resp.cost is None
+
 
 class TestEmbedPostProcess:
     async def test_normalize_l2(self):