feat: accept sampling overlay on chat() and per-source extra_body

Priority is structured injection > per-call overlay > per-source config,
and extra_body now takes part in the cache fingerprint.

tests/unit/test_openai_compat.py

@@ -445,6 +445,27 @@ class TestRequestShaping:
         )
         assert seen["response_format"] == {"type": "json_object"}
 
+    async def test_extra_body_merged_and_outranked_by_overlay(self):
+        """顺序即优先级: thinking profile → extra_body → overlay(issue #4)。"""
+        seen = {}
+
+        def handler(request):
+            seen.update(json.loads(request.content))
+            return _sse_stream(_chunk(content="x"), _chunk(usage=_USAGE))
+
+        await _complete(
+            _transport_for(handler),
+            _source(extra_body={"temperature": 0, "top_p": 0.9}),
+            overlay={"temperature": 1},
+        )
+        assert seen["temperature"] == 1  # 调用级覆盖配置级
+        assert seen["top_p"] == 0.9  # 未被顶掉的配置级键保留
+
+    async def test_extra_body_cannot_break_governed_keys(self):
+        """治理键由 payload 骨架拥有;extra_body 的保护键在构造期已被拦下。"""
+        with pytest.raises(ValueError, match="stream"):
+            _source(extra_body={"stream": False})
+
 
 class TestErrorTranslation:
     @pytest.mark.parametrize(