fix: close the failure modes review found in the new code

Three of them were the same shape as the bug this branch exists to fix:
something goes wrong, the library swallows it, and the caller is left
with a number that means the opposite of what happened.

The throttle key had no source in it. Five sources on one model is the
normal case here, so the first one to break would warn once and silence
the other four for the life of the process, and the message never said
which gateway to look at.

An unknown verdict in a cached entry threw away the whole response. The
rehydrator tolerates unknown fields but not unknown values of a known
field, so two library versions sharing a Redis would each invalidate
the other's entries: halved hit rate, and the only log line says the
cache rebuild failed. A purely observational field should not be able
to void a response whose content is intact.

Normalising for telemetry now degrades instead of raising, both for a
bare string and for a value outside the domain. Either one used to
reach the same except and cost the whole row, which is exactly how
1.3.0 lost nineteen calls without anyone noticing.

tests/e2e/test_thinking_live.py

@@ -413,6 +413,31 @@ class TestOtherProviders:
         )
         assert len(offs) == len(obs), f"{provider} 关闭方向未满足: {obs}"
 
+    async def test_qwen_enabled_is_observed(self):
+        """设计 §14 验收: qwen 开启档必须裁定为 `OBSERVED`,不是 `UNKNOWN`。
+
+        本条是三态裁定的**跨供应商对照组**: MiniMax 这一路两个信号都可能缺失
+        (非流式档整片 `UNKNOWN`),若只按它调判据,很容易把"观测不到"当成常态;
+        qwen 在同一网关同一 key 上照常返回推理信号(findings 2026-08-25 §2),
+        故这里能且必须要求正面结论——它一旦掉成 `UNKNOWN`,说明的是库的组装路径
+        丢了信号,而不是上游行为变了。
+        """
+        matrix, provider, model = "L6b", "qwen", "qwen3.7-plus"
+        desc = f"{provider} enable_thinking=True"
+        try:
+            obs = await _run_rounds(_ROUNDS, provider=provider, model=model, enable_thinking=True)
+        except (AllSourcesExhausted, SourceDeadError, TransientError) as exc:
+            _skip_if_unreachable(exc, matrix, desc)
+        ons = [o for o in obs if _reasoning_on(o)]
+        _record(
+            matrix,
+            desc,
+            "PASS" if len(ons) * 2 > len(obs) else "FAIL",
+            f"{len(ons)}/{len(obs)} 轮观测到推理(OBSERVED)",
+            obs,
+        )
+        assert len(ons) * 2 > len(obs), f"{provider} 开启方向要求多数轮 OBSERVED: {obs}"
+
 
 class TestCapabilityDrift:
     """L8 漂移哨兵: 能力表过期是必然事件,这里是它的过期告警。"""