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_client.py

@@ -202,6 +202,33 @@ class TestModelFingerprint:
         assert plain != tuned
         assert tuned.startswith("qwen-max|")  # 旧字面量仍是前缀,便于人眼辨认
 
+    def test_enable_thinking_changes_fingerprint(self):
+        """issue #5 配套: thinking 一旦真正改变请求体,就必须进缓存身份。
+
+        否则"关掉推理后重启"会读到开着推理时缓存的旧响应——issue #4 为
+        temperature 写过逐字相同的理由。
+        """
+        from polygateway.client import build_model_fingerprint
+
+        plain = build_model_fingerprint([_source()])
+        off = build_model_fingerprint([_source(enable_thinking=False)])
+        on = build_model_fingerprint([_source(enable_thinking=True)])
+        assert len({plain, off, on}) == 3
+
+    def test_extra_body_only_fingerprint_is_byte_identical_to_before(self):
+        """只配 extra_body、不表态 thinking 的存量源不得触发冷启动。
+
+        字面量在此硬编码: 这条断言的价值全在"逐字相同",改实现时必须先看见它红。
+        """
+        import hashlib
+        import json
+
+        from polygateway.client import build_model_fingerprint
+
+        mark = json.dumps(["qwen-max", {"temperature": 0}], sort_keys=True, ensure_ascii=False)
+        expected = "qwen-max|" + hashlib.sha256(mark.encode("utf-8")).hexdigest()
+        assert build_model_fingerprint([_source(extra_body={"temperature": 0})]) == expected
+
     def test_source_rename_does_not_change_fingerprint(self):
         """指纹按 (model, extra_body) 而非源名: 改名不该误触全量冷启动。"""
         from polygateway.client import build_model_fingerprint