fix: revive the reasoning verdict as an enum, not a bare string

asdict keeps the enum and json.dumps writes it as a string because
StrEnum is a str subclass, but nothing turns it back on the way in, so
a cache hit returned a plain str where the annotation promised an enum.
Verified end to end rather than assumed from the subclass relation.

A value outside the domain now raises inside the existing guard and the
call falls back to source, which is the right direction for a poisoned
or stale cache entry. Entries written before this column existed still
replay: the guard checks for the key first, and a test pins that, since
turning it into an unconditional conversion would quietly turn every
pre-upgrade entry into a permanent miss.

tests/unit/test_cache.py

@@ -10,7 +10,7 @@ from polygateway.backends.memory.cache import InMemoryCache
 from polygateway.errors import ResultInvalidError, TransientError
 from polygateway.middleware.cache import CacheMW, build_cache_key, digest_messages
 from polygateway.middleware.telemetry import TelemetryEmitter
-from polygateway.types import ChatRequest, LLMResponse, SourceConfig
+from polygateway.types import ChatRequest, LLMResponse, SourceConfig, ThinkingObservation
 
 _MSGS = [{"role": "user", "content": "hi"}]
 
@@ -249,6 +249,52 @@ class TestObservabilityFieldsOnHit:
         assert hit.cached_prompt_tokens is None and hit.model_reported is None
 
 
+class TestThinkingObservationRehydration:
+    """issue #16/#17: 命中回放必须复活成枚举实例,而不是 JSON 里的裸 str。
+
+    裸 str 与字段注解分叉,下游拿 `resp.thinking_observation is
+    ThinkingObservation.OBSERVED` 判等会在缓存命中路径上静默为 False。
+    """
+
+    async def test_hit_replays_enum_instance_not_bare_str(self):
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        terminal = _Terminal(_resp(thinking_observation=ThinkingObservation.OBSERVED))
+        await mw(ChatRequest(messages=_MSGS), terminal)
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.cache_hit is True and terminal.calls == 1
+        assert isinstance(hit.thinking_observation, ThinkingObservation)
+        assert hit.thinking_observation is ThinkingObservation.OBSERVED
+
+    async def test_illegal_value_falls_back_to_source(self):
+        """污染值(旧版本写入或人为篡改)按未命中回源,不得复活出域外取值。"""
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        key = build_cache_key("m", _MSGS, "proj", None)
+        poisoned = dataclasses.asdict(_resp(content="poisoned"))
+        poisoned["thinking_observation"] = "bogus"
+        poisoned.pop("structured_data", None)
+        await backend.set(key, json.dumps(poisoned), 3600)
+        terminal = _Terminal(_resp())
+        resp = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert terminal.calls == 1 and resp.cache_hit is False
+        assert resp.content == "cached"  # 回源结果,不是被污染的那条
+
+    async def test_legacy_entry_without_key_rehydrates_to_default(self):
+        """升级前写入的条目没有该键,必须照常复活并落到默认 UNKNOWN。"""
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        key = build_cache_key("m", _MSGS, "proj", None)
+        legacy = dataclasses.asdict(_resp(content="legacy"))
+        legacy.pop("thinking_observation")
+        legacy.pop("structured_data", None)
+        await backend.set(key, json.dumps(legacy), 3600)
+        terminal = _Terminal(_resp())
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.content == "legacy" and terminal.calls == 0
+        assert hit.thinking_observation is ThinkingObservation.UNKNOWN
+
+
 class _BrokenBackend:
     async def get(self, key):
         raise ConnectionError("redis down")