feat: add cache_salt dimension to LLM response cache

tests/unit/test_redis_cache.py

@@ -109,6 +109,18 @@ async def test_different_models_different_keys(
     assert await cache.get("claude-3-opus", MESSAGES) is None
 
 
+@pytest.mark.asyncio
+async def test_salt_changes_key(fake_redis: object) -> None:
+    """cache_salt 进入键：不同 salt 产生不同键，None 保持旧键结构。"""
+    cache = RedisResponseCache(redis=fake_redis, ttl_s=None)
+    k_none = cache._build_key("m", [{"role": "user", "content": "x"}], None)
+    k_e1 = cache._build_key("m", [{"role": "user", "content": "x"}], "run:e1")
+    k_e2 = cache._build_key("m", [{"role": "user", "content": "x"}], "run:e2")
+    assert k_none != k_e1
+    assert k_none != k_e2
+    assert k_e1 != k_e2
+
+
 @pytest.mark.asyncio
 async def test_graceful_degradation_on_error() -> None:
     """Redis 不可用时静默降级：get→None，set→不报错。"""