test: apply evidence-based live checks without hiding regressions

tests/unit/test_client.py

@@ -1340,3 +1340,25 @@ async def test_managed_conflict_returns_half_open_probe_and_permit():
         await gate.release_probe(next_entry)
     finally:
         await client._transport.aclose()
+
+
+async def test_synthetic_runtime_protocol_and_legacy_call_signatures():
+    """合成 Protocol 仅证明本库兼容契约，不冒充缺失下游实际验收。"""
+    import inspect
+    from typing import Protocol, runtime_checkable
+
+    @runtime_checkable
+    class Caller(Protocol):
+        """旧调用点只依赖 chat 协议。"""
+
+        async def chat(self, messages, **kwargs): ...
+
+    client = GatewayClient.from_env("LLM", env=_ENV)
+    try:
+        assert isinstance(client, Caller)
+        signature = inspect.signature(client.chat)
+        signature.bind([], session_id="session", parent_call_id="step")
+        signature.bind([], session_id="session", cache_salt="epoch-1")
+        assert client._transport._clients == {}
+    finally:
+        await client.aclose()