fix: address the independent verification findings (issue #5, #6)

The verifier caught that the disable-direction evidence only proved "no
regression", not "actually took effect": on M3 the disabled runs and the
no-opinion baseline are identically distributed, because that model does
not reason by default anyway. So the disable runs alone cannot rule out
the very failure mode issue #5 is about -- the parameter being silently
dropped upstream. The bogus-value experiment that does rule it out was
sitting in the findings document instead of the test suite; it is now
case L3b, and the L3 assertion that could never fail is gone.

Also from the review: the e2e helper caught bare Exception, which would
have disguised a library bug as an unavailable source, exactly the
silence the reporting discipline exists to prevent; the unregistered
model warning fired on every request instead of once per source; and the
transport caught ValueError broadly enough to mislabel unrelated errors,
now narrowed to a dedicated ThinkingUnsupportedError.

The design and plan still described the original judgement criteria,
which the measurements had already overturned. Both now match what the
tests actually do, and the design no longer claims the only new failure
surface is the openai one -- dissect configures MiniMax-M2.7 with
ENABLE_THINKING=false and will fail at assembly, which has to be
coordinated before this merges.

src/polygateway/providers.py

@@ -87,6 +87,17 @@ DEFAULT_PROFILES: Mapping[str, ProviderProfile] = MappingProxyType(
 )
 
 
+class ThinkingUnsupportedError(ValueError):
+    """推理开关无法满足: 形态未知或该模型不支持该方向(issue #5)。
+
+    是 `ValueError` 的子类而非 `errors.py` 四分类之一——它描述的是**配置**
+    不可满足(装配期就该炸),不是一次调用的运行时失败。transport 在请求期
+    捕获它并翻译为 `RequestRejectedError` 再进四分类。单列一个类型是为了让
+    捕获点能精确到它,而不是宽catch 整个 `ValueError`(那会把序列化等无关
+    错误误贴成"推理开关无法满足")。
+    """
+
+
 @dataclass(frozen=True)
 class ThinkingCapability:
     """某个**具体模型**能否关闭推理(issue #5);登记必须附实测证据与日期。
@@ -162,6 +173,7 @@ def resolve_thinking(
     enable_thinking: bool | None,
     *,
     model: str,
+    warn_unregistered: bool = True,
 ) -> Mapping[str, Any]:
     """三态 + 两层能力 → 请求体注入片段;不可满足时 ValueError。
 
@@ -172,6 +184,9 @@ def resolve_thinking(
 
     `model` 只用于错误与告警文案: 报错能定位到具体模型才有可操作性,而
     `capability` 为 None(未登记)时无从从别处取得模型名。
+
+    `warn_unregistered=False` 供请求热路径去重用: 装配期已经喊过一次,逐次
+    调用再喊只会刷屏。判定结果不受此参数影响。
     """
     # Phase 1: 调用方不表态 —— 与 False 严格区分,用模型默认档
     if enable_thinking is None:
@@ -180,31 +195,36 @@ def resolve_thinking(
     direction = "thinking_on" if enable_thinking else "thinking_off"
     # Phase 2: 形态未知 —— 提供了开关却不知道怎么发,静默放行就是欺骗调用方
     if slot is None:
-        raise ValueError(
+        raise ThinkingUnsupportedError(
             f"provider {profile.name!r} 的 {direction} 形态未知(模型 {model!r}): "
             f"本库不知道该 provider 如何表达这一档。请用 register_provider 注册形态,"
             f"或改用 SourceConfig.extra_body 直接下发供应商参数"
         )
     # Phase 3: 能力未登记 —— 新模型上线不该被库挡住,但也不该假装成功
     if capability is None:
-        logger.warning(
-            "模型 {} 的推理能力未登记,按 provider {} 的形态尽力注入 {};"
-            "若该模型实际不支持这一档,本次设置将静默失效。实测后请用 register_capability 登记",
-            model,
-            profile.name,
-            dict(slot),
-        )
+        if warn_unregistered:
+            _warn_unregistered(model, profile, slot)
         return slot
     # Phase 4: 明确不支持关闭 —— 调用方要的是"不推理"的语义保证,给不了必须说
     if enable_thinking is False and not capability.can_disable:
-        raise ValueError(
+        raise ThinkingUnsupportedError(
             f"模型 {model!r} 无法关闭推理,enable_thinking=False 无法满足: "
             f"{capability.evidence}。该模型的推理是固有属性,任何参数都关不掉——"
-            f"若实验需要关闭思维链,请换用支持关闭的模型"
+            f"需要关闭思维链请换用支持关闭的模型"
         )
     return slot
 
 
+def _warn_unregistered(model: str, profile: ProviderProfile, slot: Mapping[str, Any]) -> None:
+    logger.warning(
+        "模型 {} 的推理能力未登记,按 provider {} 的形态尽力注入 {};"
+        "若该模型实际不支持这一档,本次设置将静默失效。实测后请用 register_capability 登记",
+        model,
+        profile.name,
+        dict(slot),
+    )
+
+
 def get_provider(
     name: str, *, registry: Mapping[str, ProviderProfile] | None = None
 ) -> ProviderProfile: