feat: carry the new observability fields through retry and cache

tests/unit/test_cache.py

@@ -150,6 +150,49 @@ class TestCacheFlow:
         assert terminal.calls == 2
 
 
+class TestObservabilityFieldsOnHit:
+    """issue #3 决策 B1: 命中行原样回放,与 model/prompt_tokens 同一口径。"""
+
+    async def test_fields_replayed_on_hit(self):
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        terminal = _Terminal(
+            _resp(cached_prompt_tokens=64, model_reported="MiniMax-Text-01-250321")
+        )
+        await mw(ChatRequest(messages=_MSGS), terminal)
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.cache_hit is True
+        assert hit.cached_prompt_tokens == 64
+        assert hit.model_reported == "MiniMax-Text-01-250321"
+
+    async def test_legacy_cache_entry_without_new_keys_rehydrates(self):
+        """旧格式条目(无这两个键)必须照常重建为 None,不得抛异常回源。"""
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        key = build_cache_key("m", _MSGS, "proj", None)
+        legacy = {
+            "content": "legacy",
+            "thinking": "",
+            "model": "m",
+            "provider": "p",
+            "prompt_tokens": 1,
+            "completion_tokens": 2,
+            "latency_ms": 30,
+            "ttft_ms": 5.0,
+            "max_inter_token_ms": 2.0,
+            "cache_hit": False,
+            "call_id": "orig",
+            "source_name": "s1",
+            "cost": None,
+            "usage_source": "measured",
+        }
+        await backend.set(key, json.dumps(legacy), ttl_s=100)
+        terminal = _Terminal(_resp())
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.content == "legacy" and terminal.calls == 0  # 真的走了缓存
+        assert hit.cached_prompt_tokens is None and hit.model_reported is None
+
+
 class _BrokenBackend:
     async def get(self, key):
         raise ConnectionError("redis down")