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_openai_compat.py

@@ -13,12 +13,14 @@ from polygateway.errors import (
     SourceDeadError,
     TransientError,
 )
+from polygateway.middleware.telemetry import TelemetryEmitter
+from polygateway.pricing import ModelPrice, PricingTable
 from polygateway.transports.openai_compat import (
     OpenAICompatTransport,
     _iter_sse_deltas,
     _sse_data_payload,
 )
-from polygateway.types import SourceConfig
+from polygateway.types import ChatRequest, LLMResponse, SourceConfig
 
 
 def _source(**overrides):
@@ -71,6 +73,53 @@ async def _complete(transport, source, *, stream=True, overlay=None):
     )
 
 
+# 单价刻意取"输出贵于输入"的真实形态: est_tokens 兜底把整估值塞进 completion
+# 时,虚高才显形(设计 §1 的 26 倍算例即此单价)。
+_PRICING = PricingTable({"qwen-max": ModelPrice(input_per_1m=1.0, output_per_1m=8.0)})
+
+
+class _MemoryRecorder:
+    def __init__(self):
+        self.rows = []
+
+    async def record_llm_call(self, **fields):
+        self.rows.append(fields)
+
+
+async def _recorded_cost(result, source):
+    """把 transport 产物走一遍真实计费路径,返回落库的 cost。
+
+    `unavailable` → cost=None 的判定在 `TelemetryEmitter` 里(设计 §3.2 #5),
+    直接调 `PricingTable.cost` 对 `0/0` 只会得到 `0.0`——那正是本组用例要防的
+    假金额,故断言必须穿过 emitter 而不是单测 pricing。
+    """
+    recorder = _MemoryRecorder()
+    response = LLMResponse(
+        content=result.content,
+        thinking=result.thinking,
+        model=source.model,
+        provider=source.provider,
+        prompt_tokens=result.prompt_tokens,
+        completion_tokens=result.completion_tokens,
+        latency_ms=1,
+        ttft_ms=result.ttft_ms,
+        max_inter_token_ms=result.max_inter_token_ms,
+        cache_hit=False,
+        call_id="cid-1",
+        source_name=source.name,
+        usage_source=result.usage_source,
+    )
+    await TelemetryEmitter(recorder, pricing=_PRICING).emit_attempt(
+        request=ChatRequest(messages=[{"role": "user", "content": "hi"}]),
+        source=source,
+        call_id="cid-1",
+        latency_ms=1,
+        response=response,
+        error=None,
+    )
+    return recorder.rows[0]["cost"]
+
+
 class TestSsePureFunctions:
     def test_data_payload_filters_noise(self):
         assert _sse_data_payload("") is None
@@ -129,13 +178,21 @@ class TestStreamHappyPath:
         assert result.content == "answer"
         assert result.thinking == "hmm"
 
-    async def test_usage_missing_falls_back_to_est(self):
+    async def test_usage_missing_is_unavailable_with_null_cost(self):
+        """usage 帧缺失 → 0/0 + unavailable + cost NULL(设计 §3.2 #3)。
+
+        改前拿 `est_tokens` 当实测并整估值塞 completion,同一条调用记成
+        `0/4000` → cost 0.032(设计 §1 的 26 倍虚高)。
+        """
+
         def handler(request):
             return _sse_stream(_chunk(content="ok"))
 
-        result = await _complete(_transport_for(handler), _source(tpm=1000, est_tokens=333))
-        assert result.usage_source == "estimated"
-        assert result.prompt_tokens == 0 and result.completion_tokens == 333
+        source = _source(tpm=1000, est_tokens=4000)
+        result = await _complete(_transport_for(handler), source)
+        assert result.usage_source == "unavailable"
+        assert result.prompt_tokens == 0 and result.completion_tokens == 0
+        assert await _recorded_cost(result, source) is None
 
 
 class TestMissingDoneSemantics:
@@ -146,12 +203,28 @@ class TestMissingDoneSemantics:
         with pytest.raises(TransientError, match="missing_done|truncated"):
             await _complete(_transport_for(self._no_done_handler), _source())
 
-    async def test_salvage_policy_keeps_content_as_estimated(self):
-        result = await _complete(
-            _transport_for(self._no_done_handler), _source(missing_done="salvage")
-        )
+    async def test_salvage_with_usage_frame_degrades_to_estimated(self):
+        """打捞且收到 usage 帧: 数字真实、可信度降级 → estimated 且照常计费。"""
+        source = _source(missing_done="salvage", tpm=1000, est_tokens=4000)
+        result = await _complete(_transport_for(self._no_done_handler), source)
         assert result.content == "partial"
-        assert result.usage_source == "estimated"  # 打捞路径强制 estimated
+        assert result.usage_source == "estimated"
+        assert result.prompt_tokens == 11 and result.completion_tokens == 7
+        assert await _recorded_cost(result, source) == pytest.approx(
+            11 / 1_000_000 * 1.0 + 7 / 1_000_000 * 8.0
+        )
+
+    async def test_salvage_without_usage_frame_stays_unavailable(self):
+        """打捞且 usage 帧缺失: 0/0 不得被洗成 estimated,否则算出假的 0.0(设计 §3.2 #4)。"""
+
+        def handler(request):
+            return _sse_stream(_chunk(content="partial"), done=False)
+
+        source = _source(missing_done="salvage", tpm=1000, est_tokens=4000)
+        result = await _complete(_transport_for(handler), source)
+        assert result.content == "partial"
+        assert result.usage_source == "unavailable"
+        assert await _recorded_cost(result, source) is None
 
     async def test_early_eof_always_transient_even_under_salvage(self):
         def handler(request):