feat: model the thinking switch as shape plus capability (issue #5)

enable_thinking=False was a no-op for minimax and openai sources: both
profiles had empty dicts on each side, so the payload update injected
nothing while the caller believed reasoning had been turned off. A
downstream project was blocked on exactly this.

The root cause is that an empty dict meant two different things -- "no
injection needed" and "we do not know how this provider spells it" --
and that a provider-level table cannot express what turned out to be a
per-model property. Live testing showed MiniMax-M3 can disable
reasoning via reasoning_effort while M2.7 and M2.5 cannot be disabled
at all, which two external registries independently confirm.

So the shape stays at provider level and a capability table joins it at
model level. Unknown, unsupported and no-opinion are now three distinct
values, and resolve_thinking is the single place they meet: it raises at
assembly time when a model cannot honour the request, warns and injects
for unregistered models, and injects silently otherwise. Every registered
capability carries the evidence it was derived from.

enable_thinking also joins the cache fingerprint, since it now really
does change the request body.

tests/unit/test_config.py

@@ -460,6 +460,36 @@ class TestCrossFieldInvariants:
         with pytest.raises(ValueError, match="lease_ttl_s"):
             GatewayClient.from_settings(dataclasses.replace(base, lease_ttl_s=1.0))
 
+    # —— 推理开关的装配守卫(issue #5)——
+
+    def _thinking_sources(self, provider, model, enable_thinking):
+        base = self._base()
+        src = dataclasses.replace(
+            base.sources[0], provider=provider, model=model, enable_thinking=enable_thinking
+        )
+        return dataclasses.replace(base, sources=(src,))
+
+    def test_model_that_cannot_disable_thinking_fails_at_assembly(self):
+        """M2.x 关不掉推理: 配了 false 必须当场炸,而不是装出一个骗人的 client。"""
+        settings = self._thinking_sources("minimax", "MiniMax-M2.7", False)
+        with pytest.raises(ValueError, match="MiniMax-M2.7"):
+            GatewayClient.from_settings(settings)
+
+    def test_unknown_thinking_shape_fails_at_assembly(self):
+        """provider=openai 是任意兼容厂商的兜底段名,形态未知即报错并指路。"""
+        settings = self._thinking_sources("openai", "kimi-k3", False)
+        with pytest.raises(ValueError, match="register_provider"):
+            GatewayClient.from_settings(settings)
+
+    def test_supported_combination_assembles(self):
+        settings = self._thinking_sources("minimax", "MiniMax-M3", False)
+        assert GatewayClient.from_settings(settings) is not None
+
+    def test_not_taking_a_position_never_trips_the_guard(self):
+        """enable_thinking=None(不干预)对任何 provider 都不该被守卫拦下。"""
+        settings = self._thinking_sources("openai", "kimi-k3", None)
+        assert GatewayClient.from_settings(settings) is not None
+
     def test_ocr_settings_cannot_wrap_invalid_gateway(self):
         """OcrSettings/EmbeddingSettings 只是包一层 GatewaySettings,自动继承同一把关。"""
         base = self._base()