fix: gate cache writes and reads on a shared cacheability verdict

Cacheable is now a strict subset of deliverable: salvaged streams,
content-less results and length/filter/unknown finishes are delivered
but never written, and the read path applies the same verdict to the
stored values before hit overrides (invalid stream_end or non-string
finish read as a miss; 1.3.9 fidelity entries fall back to the
completion's finish). Hits reset stream_end. The legacy
_has_completion_output helper is folded into _is_cacheable.

Red: 13 tests in tests/unit/test_cache.py failed before this change
(uncacheable results were written and replayed; new fields not reset).

tests/integration/test_redis_cache.py

@@ -30,6 +30,13 @@ def _resp(content="cached"):
     return LLMResponse(content, "", "m", "p", 1, 2, 30, None, None, False, "orig")
 
 
+def _const(response):
+    async def terminal(request):
+        return response
+
+    return terminal
+
+
 @pytest.fixture
 async def redis_cache():
     import redis.asyncio as aioredis
@@ -82,6 +89,23 @@ class TestRealRedisRoundtrip:
         assert first.cache_hit is False and second.cache_hit is True
         assert calls["n"] == 1
 
+    async def test_completion_verdict_fields_roundtrip(self, redis_cache, namespace):
+        """1.3.10: finish_reason/stream_end 经真实 Redis 往返;不可缓存结果不落库。"""
+        mw = CacheMW(
+            backend=redis_cache, model_fingerprint="m", default_namespace=namespace, ttl_s=60
+        )
+        from dataclasses import replace
+
+        ok = replace(_resp(), finish_reason="stop", stream_end="eof_after_finish")
+        first = await mw(ChatRequest(messages=_MSGS), _const(ok))
+        second = await mw(ChatRequest(messages=_MSGS), _const(ok))
+        assert first.cache_hit is False and second.cache_hit is True
+        assert second.finish_reason == "stop" and second.stream_end is None
+        truncated = replace(_resp(), stream_end="salvaged")
+        other = [{"role": "user", "content": "integration-salvaged"}]
+        await mw(ChatRequest(messages=other), _const(truncated))
+        assert await redis_cache.get(build_cache_key("m", other, namespace, None)) is None
+
     async def test_namespace_isolation_on_shared_instance(self, redis_cache, namespace):
         """多项目共用同一 Redis 时,namespace 不同绝不互相命中(防毒化铁律)。"""
         other = f"pgw:test:{uuid.uuid4().hex}"