test: pin the tier fallback and the assembly guard to real behaviour

Both were wired but unwitnessed: hardcoding the transport's fallback, or
blanking the source tier the assembly guard reads, left the whole unit
suite green. Cover them where the value is visible -- the bytes on the
wire for nearest-vs-error, and the assembly-time refusal that must name
low as the executable alternative.

tests/unit/test_client.py

@@ -12,6 +12,7 @@ from polygateway import (
     AllSourcesExhausted,
     GatewayClient,
     GatewaySettings,
+    RequestRejectedError,
     gather_bounded,
 )
 from polygateway.backends.memory.breaker import InMemoryGate
@@ -275,6 +276,42 @@ class TestReasoningEffortPriority:
         assert "reasoning_effort" not in body
 
 
+class TestEffortFallbackWiring:
+    """源级 `effort_fallback` 必须真的走到 `resolve_thinking`(issue #20)。
+
+    只测配置值域与纯函数是不够的: 变异实测显示把 transport 里的
+    `fallback=source.effort_fallback` 换成硬编码 `"error"`,全套单测依然全绿——
+    `nearest` 是人类明确要求实现的功能,没有这条端到端用例它会在零告警下变成
+    死代码(2026-09-05 独立验证查出)。故断言看**发出去的字节**。
+    """
+
+    def _capturing_client(self, captured, **overrides):
+        def handler(request):
+            captured.append(json.loads(request.content))
+            return _sse()
+
+        return _client(handler=handler, **overrides)
+
+    async def test_nearest_sends_the_mapped_tier(self):
+        """glm-5.3 只有 low/high/max: 请求 `medium` 等距,按"取弱"落到 low。"""
+        captured = []
+        source = _source(provider="zhipu", model="glm-5.3", effort_fallback="nearest")
+        async with self._capturing_client(captured, sources=[source]) as client:
+            await client.chat([{"role": "user", "content": "hi"}], reasoning_effort=Effort.MEDIUM)
+        assert captured[0]["reasoning_effort"] == "low"
+
+    async def test_error_fallback_refuses_the_same_request(self):
+        """默认 `error` 必须报错而不是映射: 一次静默的换档就是一笔没打招呼的账单。"""
+        captured = []
+        source = _source(provider="zhipu", model="glm-5.3")
+        async with self._capturing_client(captured, sources=[source]) as client:
+            with pytest.raises(RequestRejectedError, match="medium"):
+                await client.chat(
+                    [{"role": "user", "content": "hi"}], reasoning_effort=Effort.MEDIUM
+                )
+        assert captured == []  # 请求根本没发出去
+
+
 class TestRequestTierNormalization:
     """`chat(reasoning_effort=...)` 是公共入口,裸字符串必须在此归一(issue #20)。